Tropical Freshwater Fish (Care Guides)

12 Aggressive Freshwater Fish: Temperament and Safer Care

Trevor Aquarium Basics, Aquatic Life (info & tips), Freshwater Fish (info and tips), Tropical Freshwater Fish (Care Guides) June 16, 2023September 10, 2026aggressive fish, Aggressive Freshwater Fish

Some freshwater fish are described as “aggressive,” but that label can mean several different things. A fish may defend a territory, compete with its own species, nip long fins, guard eggs, or simply eat any tank mate small enough to swallow. Knowing which behaviour you are dealing with is the key to safer stocking.

This guide introduces 12 popular territorial, fin-nipping, or predatory freshwater fish. It is not a compatibility guarantee: individual temperament, sex, age, tank dimensions, cover, diet, and water quality all affect behaviour.

Why Freshwater Fish Become Aggressive

Territory and breeding

Many fish defend a cave, nest, feeding area, or stretch of the aquarium. Breeding pairs can become much more defensive while guarding eggs or fry. Sight breaks made with plants, wood, and rock can help, but they do not make an undersized or poorly matched community safe.

Predation is not the same as aggression

A leopard bush fish quietly swallowing a small tetra is behaving as a predator, not necessarily attacking out of hostility. Choose tank mates by adult size—not the size at which they are sold—and remember that a surprisingly wide mouth can swallow deep-bodied prey.

Crowding, stress, and an unsuitable group size

Poor water quality, insufficient swimming room, too few hiding places, and incompatible tank mates can increase conflict. Some social fish redirect more attention toward outsiders when kept in groups large enough to establish a stable hierarchy. Others, including adult rainbow sharks, are usually safest as the only member of their kind.

Safety first: Have a cycled spare tank or divider ready when introducing territorial fish. Remove a fish immediately if chasing is relentless, a fish cannot feed, or there is torn tissue, heavy breathing, or persistent hiding.

12 Aggressive or Predatory Freshwater Fish

1. Tiger Barb

Tiger barbs are active schooling fish with a well-earned reputation for fin nipping. Trouble is most likely in a cramped aquarium, in a group that is too small, or alongside slow fish with trailing fins.

Keep a substantial group—six is a practical minimum and more is often better—in a tank with open swimming space and planted sight breaks. Avoid bettas, fancy guppies, angelfish, and other tempting long-finned companions. A larger school can spread social pressure, but it does not guarantee peaceful behaviour.

2. Gourami

“Gourami” covers many species, so there is no single temperament or tank-size rule. Males of several species defend surface territory and may chase rivals. Honey gouramis are generally gentler than three-spot or paradise fish, while individual dwarf gouramis vary.

Research the exact species, avoid combining multiple males in limited space, and use tall plants or floating cover to break the line of sight near the surface. Do not assume that every small gourami is automatically suitable for a peaceful nano community.

3. Cichlids

Cichlids range from comparatively peaceful dwarf species to large, highly territorial predators. Aggression often intensifies when a pair forms or a parent guards fry. African rift-lake species, Central American cichlids, and South American species also have very different water and social requirements.

Do not mix cichlids simply because they look similar or share the same label at a shop. Match water chemistry, adult size, feeding strategy, and temperament. Our aggressive cichlid guide covers ten commonly kept species in more detail.

4. Pea Puffer

The tiny pea puffer (Carinotetraodon travancoricus) is curious, intelligent, and capable of persistent biting. Its small body does not make it a safe companion for shrimp or slow, long-finned fish.

A densely planted, species-focused setup is the most predictable option. Experienced keepers sometimes maintain carefully structured groups, but success depends on floor space, heavy visual cover, sex ratio, individual temperament, and close observation. Plan a varied meaty diet and never buy one as a general solution to a snail problem.

5. Leopard Bush Fish

The leopard bush fish (Ctenopoma acutirostre) is often calm with appropriately sized companions, yet it is an effective ambush predator. Small tetras, livebearer fry, and shrimp may disappear, especially after lights out.

Choose robust tank mates too large to swallow, provide shaded planted areas, and avoid boisterous species that prevent it from feeding. Its measured movement should not be mistaken for community-fish innocence.

6. Arowana

Arowanas are large surface predators, powerful jumpers, and specialist fish. The commonly kept silver arowana can grow far beyond the capacity of an ordinary home aquarium. Young fish are often sold at a size that hides the scale of their adult housing needs.

They require an exceptionally large, long enclosure with a secure lid and carefully chosen companions. Territorial conflict and predation are real risks. Before purchase, confirm the species, its legal status in your area, its adult dimensions, and a permanent—not temporary—housing plan.

7. Black Wolffish

Fish sold as black wolffish are large South American ambush predators in the genus Hoplias. Common names are inconsistent, which makes a scientific identification especially important. Adult size and care can differ greatly between species.

Use a tightly covered, spacious aquarium with sturdy filtration and cover that cannot collapse. Most community-style stocking is risky. If companions are attempted, they must be large, robust, and monitored closely; a dedicated aquarium is the safer default.

8. Betta Fish

Domestic male bettas are territorial toward other male bettas and may also react to similarly shaped or colourful fish. They flare, charge, and bite; two males should never share an undivided aquarium.

Some individuals live successfully with calm, short-finned companions in a well-planted tank, while others need to be kept alone. Female groups are not a beginner shortcut: “sororities” can produce severe bullying. A heated, filtered aquarium with gentle flow and regular water testing is essential. See our male and female betta guide for more detail.

9. Plecostomus

“Pleco” covers hundreds of suckermouth catfish species. Many are peaceful, but territorial disputes can occur over caves and food, particularly between mature bottom dwellers. Some common plecos also grow much larger than buyers expect and create a heavy waste load.

Identify the species before purchase, provide appropriately sized hides, and feed a complete species-appropriate diet rather than expecting the fish to live on tank algae. Separate individuals if one controls every cave or feeding site.

10. Oscar

Oscars (Astronotus ocellatus) are intelligent, heavy-bodied cichlids that eat smaller fish and may rearrange décor. Their social behaviour is individual: one pair may coexist, while another combination fights. Jaw locking can be courtship, a contest, or serious conflict, so it should never be ignored.

An adult needs far more space and filtration than a juvenile suggests. Use sturdy décor, protect heaters, and plan for high biological waste. Tank mates, if any, must match the Oscar’s size and water requirements, with a backup enclosure available.

11. Rainbow Shark

The rainbow shark is a cyprinid, not a true shark. Adults commonly defend caves and bottom territory, especially from fish with a similar shape or feeding zone.

Keep one rainbow shark per ordinary home aquarium, provide multiple retreats and sight barriers, and favour active midwater companions that are not easily intimidated. Avoid assuming that another “shark” or bottom feeder will provide company; it may instead be treated as a rival.

12. Red-Bellied Piranha

Red-bellied piranhas are powerful, nervous schooling predators—not the constantly frenzied attackers of film. They can injure one another when stressed, crowded, or competing for food, and smaller tank mates are unsafe.

This is a specialist species requiring a very large, secure aquarium, strong filtration, subdued cover, and a carefully managed group. Never hand-feed or deliberately provoke feeding displays. Check local laws before obtaining piranhas because possession is restricted in some jurisdictions.

How to Reduce Conflict

Research the exact species. Common names can cover fish with very different adult sizes and behaviour.
Plan around adult size. A juvenile’s current dimensions say little about its permanent enclosure.
Break sight lines. Plants, wood, and stable rockwork create separate visual territories.
Match feeding zones. Distribute suitable food so one dominant fish cannot control every meal.
Avoid overcrowding. Stocking more fish is not a general cure for aggression.
Watch after changes. Introductions, maturity, breeding, and rearranged décor can reset territories.
Keep an exit plan. A divider or cycled spare aquarium can prevent an injury from becoming fatal.

Frequently Asked Questions

What is the most aggressive freshwater aquarium fish?

There is no useful single winner. A small territorial fish can be more dangerous to its particular tank mates than a large predator housed correctly. Compare the type of aggression, adult size, space, and compatibility rather than relying on a ranking.

Can aggressive fish live in a community tank?

Some can, with carefully selected companions and enough space and structure. Others are more predictably kept alone or in a species-focused setup. No online list can guarantee that two individual fish will coexist.

Does adding more hiding space stop aggression?

It often helps by reducing visual contact and giving subordinate fish an escape route, but it cannot compensate for an undersized tank, an incompatible pairing, or an active predator.

Final Thoughts

A responsible setup starts by identifying whether a fish is territorial, social, fin-nipping, or predatory. Build the aquarium around its adult needs, choose companions conservatively, and intervene early when one fish cannot escape or feed. For a closer look at large and territorial cichlid species, continue with our aggressive cichlids guide.

Platinum Arowana: Identity, Care, Size and Legal Issues

Trevor Aquarium Basics, Aquatic Life (info & tips), Fish Care Guides, Tropical Freshwater (guides), Tropical Freshwater Fish (Care Guides) June 10, 2023September 16, 2026arowana, dragonfish, platinum Arowana

“Platinum arowana” is a trade description for an unusually pale or metallic colour form, not a separate species. Many fish sold under the name are platinum or leucistic silver arowanas (Osteoglossum bicirrhosum), but sellers may apply similar language to Asian arowanas. The distinction matters because species, adult care requirements and wildlife-trade rules are not interchangeable.

Reality check: A silver arowana can reach about 90 cm (35 in), is a powerful jumper and requires an exceptionally large, secure aquarium. This is an expert-only fish. Confirm the scientific name and legal documentation before paying for any animal advertised as “platinum.”

Platinum arowana facts at a glance

What the name means	A colour or appearance label, not a scientific species
Likely species in this guide	Silver arowana, Osteoglossum bicirrhosum
Recorded maximum length	About 90 cm (35 in) total length
Temperature	FishBase lists 24–30°C (75–86°F) for silver arowana
Feeding zone	Primarily near the surface
Experience level	Expert
Main risks	Adult size, jumping, predation, heavy waste, injury and legal misidentification

Platinum silver arowana versus Asian arowana

The silver arowana is a South American species from the Amazon, Rupununi and Oyapock systems. It has an elongated body, large scales, an upturned mouth and two barbels at the lower jaw. “Platinum” individuals have unusually pale coloration, but that appearance does not change their adult size or basic behaviour.

The Asian arowana (Scleropages formosus) is a different species from Southeast Asia. It is listed in CITES Appendix I, and international commercial trade is tightly controlled. CITES-registered captive-breeding operations use coded microchips for legal specimens. National and state rules can be stricter, so ownership that is legal in one country may be prohibited or permit-controlled elsewhere.

Do not rely on colour, a luxury price or the word “dragonfish” as identification. Ask for the scientific name, origin, breeder information, import or transfer paperwork and microchip documentation where applicable. If a seller cannot provide a consistent identity and lawful provenance, do not buy the fish.

Why are platinum arowanas expensive?

Rarity, appearance, size, provenance and collector demand can make pale arowanas expensive. However, widely repeated claims about a particular fish selling for a fixed six-figure or seven-figure price are often difficult to verify independently. An asking price, publicity story and completed documented sale are not the same thing.

Price does not guarantee health, genetics or legality. Before considering a high-value fish, obtain an independent health assessment, verify the identity and documents, and plan secure long-term housing. The aquarium, stand, filtration, electricity, food and emergency systems can also cost far more than a typical home setup.

Adult size and aquarium planning

FishBase records silver arowanas at up to 90 cm total length. A juvenile sold at 15–20 cm can therefore outgrow ordinary aquariums rapidly. The enclosure must be long and especially wide enough for an adult to turn without bending against the glass. Surface area matters because the fish spends much of its time near the top.

There is no responsible one-size-fits-all gallon number for this species. Body length, turning radius, aquascape, water depth, filtration, maintenance and individual behaviour all matter. In practice, an adult needs a custom-scale aquarium or other professionally designed enclosure measured in hundreds of gallons. Arrange the final system before acquiring the juvenile—not after it has begun to outgrow a temporary tank.

A secure top is essential

Arowanas are powerful jumpers. Use a rigid, closely fitted cover secured against the force of a large fish, while maintaining safe ventilation and service access. Protect the fish from sharp braces, exposed lamps and gaps around pipes or cables. A lightweight glass lid placed loosely on top is not adequate.

Stand, floor and electrical safety

A very large aquarium requires structural planning. Use a purpose-built level stand and have the supporting floor assessed when necessary. Protect electrical circuits with appropriate ground-fault protection, keep plugs above potential flood level, use drip loops and arrange equipment so a leak cannot send water onto power connections.

Filtration, cycling and water quality

A large predatory fish produces substantial solid and dissolved waste. Use high-capacity mechanical and biological filtration with redundancy, accessible prefilters and strong oxygenation. Flow should circulate the whole enclosure without forcing the fish to fight a jet continuously.

Complete a fishless nitrogen cycle before adding the fish. Ammonia and nitrite must remain at 0 ppm. Track nitrate and use test results to determine water-change frequency rather than relying on a fixed calendar alone. Match replacement water temperature and chemistry, and treat tap water for chlorine or chloramine.

Keep spare aeration, a plan for pump failure and enough conditioned water capacity for emergencies. A power outage or filter blockage becomes dangerous faster when one large fish represents the entire system’s waste load.

Temperature and aquarium layout

FishBase lists 24–30°C (75–86°F) for silver arowana. Choose a stable point within the appropriate range for the specific fish and its verified tank mates; avoid sudden changes. Large systems may need multiple guarded heaters or an external heating solution controlled and monitored independently.

Leave generous open swimming space. Use only stable, smooth décor that cannot topple or injure a startled fish. Darker surroundings may reduce reflections, but monitor the individual rather than assuming one background suits every fish. Keep the aquarium in a low-traffic position away from sudden impacts and vibration.

Diet and feeding

Silver arowanas are surface-oriented omnivores with a strong tendency to prey on fish and large insects. A captive diet should be varied and nutritionally controlled rather than based solely on feeder fish. Appropriate commercial carnivore pellets can form a dependable staple when the fish accepts them, supplemented with suitable prepared or frozen foods.

Avoid routinely feeding live fish. Feeder fish can introduce parasites and disease, may be nutritionally poor, and can injure the predator. Never use wild-caught prey without expert biosecurity advice. Remove leftovers quickly and adjust portions to body condition, growth and water-quality results.

Tank mates

No companion is guaranteed safe. Any fish small enough to swallow may become prey, while aggressive or fin-nipping species can injure the arowana. Large tank mates also increase the required enclosure size and filtration load. Territorial conflict may appear as fish mature even when juveniles initially coexist.

If mixed housing is attempted, it should be in a very large system with compatible water needs, sufficient visual separation and an immediately available backup enclosure. Quarantine every addition. Keeping a single arowana is often safer than building a crowded “monster fish” collection.

Health and injury risks

Daily observation is essential. Watch for repeated jumping, rubbing, loss of appetite, abnormal breathing, cloudy eyes, damaged scales, frayed fins, swelling, wounds or unusual posture. Test water immediately when behaviour changes because water-quality problems are common and can resemble disease.

Do not medicate a large display tank without a diagnosis. Drug dosing, oxygen depletion and filter damage can have serious consequences at this scale. Establish contact with an aquatic veterinarian before an emergency occurs and prepare safe handling and transport procedures for a large, powerful fish.

Breeding and mouthbrooding

Silver arowanas are paternal mouthbrooders: the male carries developing eggs and young in his mouth. Successful breeding requires mature compatible fish, exceptional space and specialised experience. Sexing from external appearance is unreliable for most keepers, and pairing large fish without a separation plan can cause serious injury.

Breeding is not a practical justification for buying a high-priced juvenile. Legal records, genetic management, animal welfare and facilities for offspring all need to be planned before pairing fish.

Frequently asked questions

Is a platinum arowana a separate species?

No. “Platinum” describes appearance. Confirm the scientific species because a platinum silver arowana and an Asian arowana have different origins and regulatory status.

Can a platinum arowana live in a 125-gallon aquarium?

A 125-gallon tank is not an adequate permanent home for a fish capable of reaching about 90 cm. Its width and turning space are especially limiting. Plan a custom adult enclosure before purchase.

Are platinum arowanas endangered?

The answer depends on the species. The silver arowana is not the same regulatory case as the Asian arowana. Asian arowana is listed in CITES Appendix I; verify current national and local law for the exact fish.

Do arowanas bring good luck?

Arowanas have cultural associations with prosperity and dragon symbolism, particularly in parts of Asia. That belief does not reduce the fish’s welfare needs or replace legal documentation and suitable housing.

Should you keep one?

For most home aquarists, no. A platinum arowana demands unusual space, secure construction, expensive life-support equipment, legal due diligence and long-term specialist care. Admiring these fish at a reputable public aquarium may be the more responsible choice.

Sources

Pregnant Neon Tetra? Eggs, Bloating, and What to Do

Trevor Aquarium Basics, Fish Care Guides, Freshwater Fish (info and tips), Tetras (Care Guides), Tropical Freshwater (guides), Tropical Freshwater Fish (Care Guides) December 2, 2021September 27, 2026breeding neons, neon, neon tetra

Neon tetras do not become pregnant with live babies: females develop eggs. Neon tetras (Paracheirodon innesi) lay eggs that are fertilized outside the body. However, a rounded belly alone cannot tell you whether a fish has eggs, has recently eaten, or is unwell.

Start with the fish’s behavior and your water tests. A comfortable, active female with a gently rounded abdomen needs a different response from a fish that is rapidly swelling, breathing hard, or struggling to swim.

Can neon tetras get pregnant?

Neons are egg scatterers. During spawning, a female releases eggs and a male releases sperm. There is no livebearer-style pregnancy or predictable “due date.” The more accurate term for a female carrying developed eggs is gravid.

The University of Florida’s guide to ornamental fish reproduction describes tetras as generally scattering eggs and sperm over underwater structures, without parental care. Adults may eat the eggs, so successful spawning in a community tank does not necessarily produce surviving fry.

Eggs, a full stomach, or illness?

Compare the fish with its usual appearance, ideally before feeding and from both the side and above. Adult females are often fuller-bodied than males, but body shape and the curve of the blue stripe are clues, not reliable pregnancy tests.

What you notice	What it may mean	Best next step
Gradual, fairly even fullness; normal appetite and swimming	Normal female body shape or developing eggs	Observe without repeatedly catching or isolating the fish.
Belly looks fuller immediately after a meal	Recent feeding may explain the change	Review portion size and compare again before the next feeding.
Increasing or uneven swelling; appetite loss or unusual hiding	A health problem needs consideration	Test water and seek advice from a veterinarian who treats fish if it persists or worsens.
Raised scales, severe swelling, gasping, or loss of balance	Urgent warning signs	Address water quality promptly and contact a fish veterinarian.

Fluid buildup can produce a swollen abdomen and raised, pinecone-like scales. As the Merck Veterinary Manual explains, fish diseases can have overlapping signs. “Dropsy” describes a condition rather than identifying its cause. A photograph or swollen belly does not justify choosing an antibiotic, salt treatment, or parasite medication.

What to do when a neon tetra looks swollen

Check the whole aquarium. Are other fish breathing quickly, hiding, or refusing food? A shared change makes environmental trouble especially important to investigate.
Test the water. Check ammonia, nitrite, nitrate, pH, and temperature. Ammonia and nitrite should be undetectable in an established aquarium. Our aquarium test kit guide explains which measurements different kits cover.
Correct problems gently. Use conditioned, temperature-matched replacement water for an appropriate partial water change, and check that filtration and surface movement are working. Do not abruptly change pH or hardness to encourage spawning.
Keep a short record. Note appetite, breathing, swimming, test results, and whether swelling changes. Clear photos taken under similar lighting help show progression.
Escalate when needed. Rapid deterioration, raised scales, or difficulty swimming deserves prompt veterinary advice. See our guide to a fish swimming sideways for environmental checks while arranging help.

If the female otherwise behaves normally, keep ordinary maintenance and modest, balanced meals consistent. She does not need a cramped breeding box simply because she looks round. Never squeeze her abdomen to “release” eggs.

If you want to breed neon tetras

Breeding is a separate project, not a treatment for swelling. Prepare space for growing fry and an appropriate first-food culture before attempting it. A spare tank is only useful when its water quality and filtration are ready for fish.

A practical beginner preparation is a covered 10-gallon breeding aquarium with a mature, gently running sponge filter and fine-leaved plants or a spawning mop. This is a planning suggestion, not a claim that every successful breeder uses that size. Keep the community tank’s normal care routine intact.

Neon breeding commonly uses very soft, acidic water and subdued light. In his firsthand breeding account, John Robertson describes success around 75–78°F and emphasizes light-sensitive eggs and fry. Those observations are not a reason to suddenly acidify an established tank. Acclimate fish carefully; very low buffering makes pH harder to manage. Our RO and tap-water mixing guide explains why both hardness and buffering need attention.

Remove adults after spawning if you intend to protect eggs. Avoid bright inspection lights. Newly hatched larvae initially use their yolk reserves; once free-swimming, they need food small enough to swallow, such as suitable infusoria or cultured Paramecium, before progressing to larger foods. Tiny, measured feedings and careful waste removal matter more than adding large quantities of food.

Common questions

How long is a neon tetra pregnant?

It is not pregnant in the livebearer sense. A rounded female does not have a dependable countdown to spawning. Do not wait for a supposed due date if swelling is worsening or the fish is unwell.

Does chasing mean spawning is about to happen?

Not necessarily. Chasing alone cannot confirm fertilization or even that you have correctly identified a male and female. Watch for persistent harassment and give fish cover rather than assuming all chasing is courtship.

Can a female lay eggs without a male?

A female can release unfertilized eggs, but those eggs will not develop into fry. Finding eggs does not prove that fertilization occurred.

Why are there no babies in my community tank?

Eggs may be unfertilized, eaten, or lost before you notice them. Fry also need suitable food and protection from filter intakes and tank mates. A full-bellied female by itself is not evidence that any eggs were laid.

40-Gallon Breeder Tank: Dimensions, Setup, and Stocking

Trevor Aquarium Basics, Aquatic Life (info & tips), Equipment (info & tips), Fish Care Guides, Tropical Freshwater Fish (Care Guides) November 15, 2021September 27, 202640 gallon tank, breeder tank

A 40-gallon breeder tank is a wide, relatively shallow aquarium with a nominal footprint of 36 by 18 inches. The extra front-to-back space makes it useful for planted layouts, carefully chosen community fish, and some breeding projects. “Breeder” describes the tank’s proportions; it does not mean every fish can breed or live comfortably in it.

Before buying, check the exact outside dimensions, the stand requirements, and the adult size of your intended fish. A 40-gallon breeder is different from a narrower 40-gallon aquarium.

40-gallon breeder dimensions and filled weight

The familiar nominal size is 36 inches long × 18 inches wide × 16 inches high. Frames and manufacturing differences change the actual measurements, so do not buy a tightly fitting cabinet or lid from that shorthand alone.

For example, Aquarium Masters lists its AM11040 breeder at 36.5 × 18.25 × 16.88 inches, with an empty weight of 52.8 pounds. Other brands and versions differ. Measure the specific aquarium, including its frame, and confirm the lid opening separately.

Forty US gallons of freshwater weigh approximately 334 pounds. Add the glass, substrate, rocks, equipment, and any water held outside the display. A complete setup can weigh well over 400 pounds. Water displaced by decorations reduces the actual water volume, but that is not a reason to use an undersized stand.

Use a purpose-built aquarium stand approved for the tank’s footprint and total load. Follow the aquarium manufacturer’s support instructions, keep it level, and leave no unsupported overhang. Do not assume an ordinary dresser is suitable or add foam beneath a framed aquarium unless its manufacturer calls for it.

Current tank options worth comparing

These are product options based on published specifications, not hands-on test results. Check the exact model, local availability, shipping condition, and what is included before ordering.

Aqueon Standard Glass 40 Breeder: a conventional framed glass option for a separately chosen filter, light, and lid. It remains listed by US retailers including Petco. Use Aqueon’s product information and the seller’s exact dimensions to confirm fit; do not confuse it with another 40-gallon shape.
Aquarium Masters 40 Breeder, AM11040: a manufacturer-listed alternative with the dimensions above. Ask a local dealer about availability. The manufacturer also lists a separate non-tempered version, AM11043; these are distinct model numbers.

The earlier comparison included two SeaClear aquariums measuring 36 × 15 × 16 inches. They may suit other projects, but their narrower footprint does not make them equivalent replacements for an 18-inch-wide breeder. Compare the actual shape, not just the gallon label.

Never assume a glass panel can be drilled because someone drilled another tank of the same advertised size. Confirm the construction of the exact tank and panel with its manufacturer. Tempered glass cannot be drilled safely.

Equipment checklist

Filter: choose capacity and media space for the intended livestock. A hang-on-back or canister filter can suit a community setup; an air-driven sponge filter is useful around small fry. Protect exposed intakes and make sure water moves through the tank without exhausting the fish.
Heater and thermometer: tropical fish usually need stable heating. Size the heater using the manufacturer’s guide and the difference between room temperature and target water temperature. Check the thermometer rather than trusting the dial alone.
Lid: allow room for equipment while closing escape gaps. Match the actual frame and openings, not merely a “36-inch” label.
Light and timer: choose light for the plants or corals you will keep. A fish-only tank does not need an intense plant or reef fixture.
Water-care supplies: conditioner for chlorine and chloramine, ammonia/nitrite/nitrate tests, a thermometer, siphon, and dedicated aquarium buckets.
Safe installation: use drip loops and appropriate electrical protection. Keep plugs and power strips away from water and follow each device’s immersion instructions.

Allow space behind the stand for the filter and hoses, and above it for opening the lid. Place the aquarium away from direct sun, drafts, and busy routes where it could be struck.

Set up and cycle before adding fish

Inspect the tank and stand, then install them according to the manufacturer’s instructions. Arrange stable hardscape and suitable substrate before filling.
Fill with conditioned water, start the filter, and establish the temperature required by your planned livestock. Check for leaks and proper circulation.
Use a fishless cycling method with an aquarium-specific ammonia source and follow that product’s dosing instructions. Do not add household cleaners or dose ammonia into a tank containing animals.
Test ammonia, nitrite, and pH during cycling, and track nitrate as the system develops. Wait until the filter can process the method’s prescribed ammonia dose and both ammonia and nitrite return to zero before stocking.
Change water as needed to bring nitrate into a suitable range, confirm temperature and water parameters, and introduce compatible fish gradually while continuing to test.

Clear water does not prove that a tank is cycled. The aim is to establish biological filtration that processes waste, not simply to wait for dust to settle. DrTim’s fishless cycling instructions illustrate why dosing and test results must be considered together; its doses apply to its own products, not every ammonia solution.

Stocking ideas for a 40-gallon breeder

Start with a theme and research each species’ adult size, temperature, hardness, social needs, and temperament. There is no reliable “one inch of fish per gallon” rule. A wider footprint provides useful room, but it does not overcome incompatible water needs or aggression.

Small-fish community: one suitably sized school of small tetras or rasboras can be the focus, with a compatible group of smaller bottom-dwelling fish if the substrate, temperature, and total stocking allow it. Plan group sizes before adding a centerpiece fish.
Livebearer aquarium: guppies can suit an appropriately maintained setup with suitable water hardness. Plan for rapid population growth if keeping both sexes; a breeder tank is not unlimited nursery space. Our guppy and molly guide discusses identification and responsible fry care.
Planted species-focused tank: choose one small species and build the layout around its needs, leaving open swimming areas and access for cleaning.
Breeding project: select the species first. Egg protection, gentle filtration, appropriate first foods, and separate grow-out space may matter more than the tank’s label.

Do not buy an adult Oscar, a common pleco, or a group of large cichlids for this tank on the assumption that extra filtration will make them fit. The old recommendation of an Oscar or six discus/angelfish was not a responsible general stocking plan. Large fish need species-specific housing with adequate adult swimming space and territory; Aqueon’s large-fish size chart shows how far some outgrow ordinary community aquariums.

Plants and layout

The broad base gives you room to arrange wood, rocks, and plants while leaving a swimming lane. Anubias and Java fern can be attached to hardscape with their rhizomes exposed. Rooted plants need an appropriate substrate and nutrient supply. Check mature plant size so a large sword does not crowd the whole aquarium.

For a compact red accent, see our Alternanthera reineckii Mini care guide. It needs more attention to light and nutrients than the easiest shade-tolerant plants. For attachment methods, read which aquarium glue is suitable for plants.

Can you use it for saltwater?

Yes, with a marine-specific plan. You will need suitable saltwater preparation, reliable salinity measurement, temperature control, biological filtration, and circulation suited to the animals. Coral lighting requirements depend on the corals; a freshwater plant light is not automatically a suitable reef light.

A protein skimmer can help with waste export, but it is not an automatic requirement for every possible 40-gallon setup. Likewise, strong direct flow is not appropriate for every animal. Choose livestock and equipment together, and leave room in the budget for testing and ongoing maintenance.

Water changes and ongoing care

As a starting routine for a moderately stocked freshwater aquarium, consider a weekly partial water change of about 25%, then adjust to test results, feeding, and livestock needs. In a tank actually holding 40 gallons, that is 10 gallons; substrate and decorations usually reduce the water volume.

Condition replacement water, match temperature reasonably closely, remove accumulated waste, and service the filter without replacing all biological media at once. Evaporation top-offs do not remove nitrate or other dissolved waste. For saltwater, match salinity during water changes and replace evaporation with appropriate freshwater, not more saltwater.

There is no universal rule forbidding changes larger than 30%. Water quality problems can require more, with care to avoid abrupt temperature or chemistry differences. More water volume buffers some changes, but a 40-gallon tank still needs testing, maintenance, and sensible stocking.

How to Tell If Angelfish Eggs Are Fertilized

Trevor Aquarium Basics, Aquatic Life (info & tips), Fish Care Guides, Tropical Freshwater (guides), Tropical Freshwater Fish (Care Guides) October 26, 2021September 28, 2026angel fish, angelfish, breeding angelfish

Developing angelfish eggs usually stay translucent, with a cream or amber tint, while failed eggs often turn opaque white. Color is a clue, not proof: the strongest evidence is an embryo developing inside the egg, followed by hatching. A newly laid egg can look promising before you know whether it will develop.

This guide concerns freshwater angelfish, commonly Pterophyllum scalare. Watch changes over time instead of trying to diagnose an entire clutch from one photograph.

What fertile and failed angelfish eggs look like

What you see	What it may mean	What to do
New, translucent cream or amber eggs	Potentially viable; too early for certainty	Record the laying time and observe without disturbing them
Visible embryo development inside a translucent egg	Much stronger evidence of successful fertilization	Maintain stable conditions and watch for hatching
Opaque white eggs that do not develop	Nonviable eggs; they may have been unfertilized or died afterward	Check water quality and remove accessible failed eggs carefully if needed
Cottonlike growth around an egg	Microbial growth associated with a failed egg	Address dead material and water conditions; do not assume the entire clutch is lost
Tiny larvae wriggling while still attached	The eggs have hatched	Protect them from strong flow and filter intakes

Lighting, the spawning surface and the camera’s white balance can change apparent color. Compare the same area of the clutch under similar lighting each day. Avoid repeatedly lifting a leaf or slate out of the water just to inspect it.

What does not prove fertilization?

Eggs sticking to a leaf, parents guarding them, or a second fish passing over them are not proof. Adhesion is a normal feature of angelfish eggs. A female can lay without a male, and two females may display apparently paired spawning behavior.

Sera’s explanation of eggs that fail to develop notes that an apparent fertilizing pass does not show whether fertilization actually occurred. It also identifies unsuitable water and two-female pairings as possibilities. A white egg alone cannot tell you which cause applies.

How long do angelfish eggs take to hatch?

Expect roughly 2–3 days to hatching under typical warm breeding conditions. This is an estimate, not a deadline. Temperature and the condition of the eggs affect the schedule.

After hatching, the young are often called wigglers. They remain attached or clustered and use their yolk reserves before swimming freely. Parents may move them to another surface, so an empty original spawning site does not always mean the eggs were eaten.

Aquarium Co-Op’s angelfish breeding guidance describes hatching after about 2–3 days and free swimming several days later. Have suitable first food ready before that stage; newly hatched baby brine shrimp are a common option once the fry are swimming and ready to feed.

Keep the water stable while the eggs develop

Use an established aquarium with gentle filtration and reliable temperature control. Around 80°F is a useful breeding reference, but consistency matters more than repeatedly adjusting the heater to chase a hatch time. The breeder AngelsPlus reports hatching around 60 hours at 80°F, with several more days before free swimming.

Test ammonia and nitrite. Both should read 0 ppm on an appropriate aquarium test. Detectable readings need attention; they are not acceptable simply because they seem low.
Use conditioned replacement water. Match temperature closely and avoid abrupt changes in water chemistry. Do not use a large cold-water change to force spawning.
Provide gentle circulation. Eggs need oxygenated water, but a forceful jet can damage or dislodge them.
Protect future fry. A mature sponge filter or a suitably protected intake reduces the risk of tiny fish being drawn into equipment.
Keep maintenance careful. Remove waste without siphoning up eggs or larvae. Small equipment and good visibility help.

If a water-quality problem appears, prioritize correcting it with appropriately matched water and restored filtration. Leaving harmful water untouched is not the same as keeping conditions stable.

Should you leave the eggs with the parents?

Leaving a clutch with the parents allows them to fan and clean it. It also lets you observe their behavior without transferring delicate eggs. The disadvantages are predation by tankmates and the possibility that the parents eat the clutch, especially when disturbed or inexperienced.

A quiet breeding setup gives the pair a better opportunity than a busy community aquarium. Keep routine activity calm and avoid tapping the glass or constantly shining a light at the eggs. You do not need to remove good parents automatically when the fry begin swimming.

Artificial hatching is another approach, but it requires a prepared container with matched water, temperature control and gentle aeration. Moving a clutch into an unheated bowl of fresh tap water creates more problems than it solves. Decide on the method before spawning whenever possible.

What to do when eggs turn white

A few white eggs do not mean every egg has failed. Observe whether other eggs continue developing. Parents may remove unsuccessful eggs themselves. When artificially incubating, an accessible dead egg can be removed carefully with a small pipette or similar tool, without scraping or crushing neighboring eggs.

No treatment can fertilize an unfertilized egg or restore a dead embryo. Avoid adding medication to the display aquarium just because a few eggs changed color. If an egg treatment is appropriate for a separate incubation setup, follow that specific product’s instructions and compatibility warnings; generic “drops per tank” recipes are unreliable.

Why does the whole clutch keep failing?

Keep a short record for each spawn: laying time, temperature, ammonia and nitrite results, when eggs become opaque, and whether any embryo development occurred. This is more useful than changing several things after every unsuccessful attempt.

No development across repeated clutches: confirm whether the fish are a male–female pair. Guarding behavior is insufficient to establish sex or fertility.
Development starts and then stops: examine water conditions, temperature changes, handling and circulation.
Eggs disappear: check for relocated wigglers before concluding that parents or tankmates ate them.
Repeated failure despite stable care: review hardness and breeding conditions with an experienced breeder. Avoid abrupt pH changes or adding softening chemicals without a plan.

For comparison with another egg-laying fish, see our guide to Corydoras egg development and hatching. Do not apply its timing directly to angelfish; species and temperature matter.

After hatching: prepare for fry care

Wait for free swimming and readiness to feed rather than scattering food over attached wigglers. Offer appropriately tiny food in small amounts, watch that the fry can eat it, and remove leftovers. Plan for frequent water-quality checks and enough grow-out space as the fish increase in size.

Routine adult emergency food substitutes are not a fry-feeding plan. Obtain suitable fry food ahead of time, and arrange space or homes for juveniles before deliberately raising a large batch.

Can white eggs still hatch?

Truly opaque white eggs that have stopped developing generally will not hatch. The caution is identifying them correctly: a pale but translucent egg under glare is not necessarily dead. Look for progression over time and avoid discarding the whole clutch based only on color.

How to Tell if a Molly Fish Is Pregnant: Signs and Care

Trevor Aquarium Basics, Aquatic Life (info & tips), Fish Care Guides, Mollies (Care Guides), Tropical Freshwater (guides), Tropical Freshwater Fish (Care Guides) September 29, 2021September 29, 2026breeding mollies, mollies, Molly Fish, Molly Fish Pregnant

A gradually enlarging belly in a mature female that has been with a male is a useful clue that a molly is pregnant. A dark patch near the rear of the abdomen may also be visible. No single outward sign confirms pregnancy or tells you the exact day she will give birth. Check the fish’s sex, recent history, body shape, and overall health together.

Check that your molly is female

Look at the anal fin underneath the fish, behind the belly. An adult female normally has a fan-shaped anal fin; a mature male has a narrow, rod-shaped reproductive fin called a gonopodium. Color and size alone are less dependable ways to tell them apart. Aqueon’s livebearer guide explains this difference and how these fish give birth to swimming young rather than laying eggs.

A female bought recently may already have mated at the store or breeder. Mollies can store sperm, so the absence of a male in your aquarium does not rule out a later pregnancy. This is documented for sailfin mollies in the University of Florida’s research review of the species (PDF, pages 3–4).

Pregnancy clues—and their limits

What you notice	How to interpret it
Belly grows gradually over several weeks	Supports pregnancy in an otherwise healthy female with a mating history; swelling has other possible causes.
Darker area toward the rear of the belly	A gravid spot can be visible on lighter fish. Pigmentation can hide it on black mollies.
Belly looks more squared near the rear	May occur late in pregnancy, but is not a reliable countdown to delivery.
Hiding, reduced appetite, or unusual stillness	Check water quality and health. These behaviors are not proof of labor.

Compare photographs taken from the same angle several days apart without catching or handling the fish. Balloon mollies already have a rounded body shape, making a belly-only judgment especially unreliable. Never squeeze the abdomen to check for babies.

How long are mollies pregnant?

Expect several weeks, not a precise deadline. Published estimates vary with the molly species and conditions: the Florida review reports about three to four weeks for sailfin mollies, while Aquarium Co-Op’s aquarium molly guide gives a broader 30–60-day breeding timeframe. A purchase date is not a known fertilization date.

Do not raise the temperature or make abrupt water changes to try to trigger birth. Maintain the fish’s normal, stable conditions. A female becoming distressed needs a health and water-quality check, not a revised due date.

When a swollen belly needs attention

Raised scales, sores, rapid breathing, loss of balance, or persistent refusal to eat should not be dismissed as pregnancy. Test ammonia and nitrite, check the temperature and filter, and observe whether other fish are affected. Seek an aquatic veterinarian’s advice for swelling accompanied by illness or worsening symptoms. Do not start medication solely because a fish looks round.

For a fish resting abnormally or struggling to swim, our bottom-resting fish troubleshooting guide helps organize the initial checks.

Should you separate a pregnant molly?

Choose your fry-raising plan before delivery. In a planted community aquarium, dense cover offers newborns hiding places, but adults—including their mother—may eat them. This approach does not guarantee that every baby survives.

If you want to raise more fry, prepare a separate, fully cycled nursery with gentle filtration and water conditions matching the main tank. A bare container of aquarium water is not a cycled nursery. Protect filter intakes so small fish cannot be drawn in.

Avoid repeatedly chasing a heavily pregnant fish or keeping her confined in a tiny breeding trap for days while guessing her due date. If you cannot provide suitable separate housing, improve cover in the existing aquarium and plan proper grow-out space rather than improvising an unfiltered bowl.

What to do when the babies arrive

Molly fry can take suitably fine food from birth. Offer small portions of finely crushed appropriate fish food or fry food; newly hatched brine shrimp are another option. Feed little and often, remove leftovers, and monitor water quality closely. Extra feeding can quickly overwhelm a small nursery.

In a separate breeding tank, remove the mother once delivery is finished and she can be moved without unnecessary disturbance. Use the fry’s actual size and the mouths of potential tankmates to judge when they can safely join a larger aquarium—not a fixed four- or eight-week rule. See our molly fry care guide for the next stage.

Plan for more than one brood

Decide where growing juveniles will live before trying to save a whole brood. Do not release unwanted aquarium fish outdoors. Arrange suitable homes and discuss sex separation with an experienced fishkeeper; females with a previous mating history may still produce young afterward.

For long-term housing and water requirements, continue with our molly fish care guide.

Best Fish for a 20-Gallon Tank: 3 Practical Stocking Plans

Trevor Aquarium Basics, Aquatic Life (info & tips), Beginners (info & tips), Equipment (info & tips), Fish Care Guides, Tank Set-up (Care Guide), Tropical Freshwater Fish (Care Guides) September 27, 2021September 29, 202620-gallon, fish for a 20-gallon tank

Ember tetras, pygmy corydoras, honey gouramis, and male guppies are useful starting points for a 20-gallon freshwater aquarium. They are alternatives to choose between, not a shopping list to combine. Match adult size, social needs, water chemistry, and temperament before deciding how many fish to buy.

Below are three modest stocking plans. Choose one, establish the aquarium first, and leave room to respond if a fish is bullied or the tank needs more maintenance than expected.

20-gallon long vs. 20-gallon high

Both hold a nominal 20 US gallons, but their swimming space differs. Aqueon’s standard 20 long measures 30¼ × 12½ × 12¾ inches; its 20 high measures 24¼ × 12½ × 16¾ inches (length × width × height). Other models vary; check the actual footprint. These measurements come from Aqueon’s aquarium dimensions chart (PDF).

For the small community fish discussed here, the long version provides a longer swimming route and more floor area. A taller tank does not compensate for insufficient swimming length, and substrate and decorations reduce the water volume in either shape.

Three separate stocking plans

Plan	Fish	Conditions and layout
Small centerpiece community	1 honey gourami + 10 ember tetras	About 76–78°F; planted cover, gentle flow, and open swimming space. Soft to moderately hard water suited to both species.
Small schooling community	10 ember tetras + 8 pygmy corydoras	About 75–77°F; preferably a 20 long, with smooth substrate, plants, and accessible feeding areas.
Hard-water livebearer tank	6 male guppies	About 76–78°F; suitably hard, alkaline water, planted sight breaks, and monitoring for chasing.

These are suggested starting designs, not universal maximum capacities or guaranteed compatible groups. They assume an established filter, appropriate food, and regular maintenance. Do not merge the plans or automatically add another bottom-dwelling group. Check the conditions in which your supplier keeps the fish and confirm that your tap water suits the chosen species.

Why these fish work

Ember tetras: one coordinated group

These small orange fish reach about 0.8 inch and suit peaceful planted aquariums. Keep a group rather than scattering two or three individuals among several tetra species. Ten gives you a useful shoal while leaving space for one compatible addition in the plans above. Choose food small enough for their mouths. Aquarium Co-Op’s ember tetra guide describes their social needs and preference for gentle filtration.

Honey gourami: a modest centerpiece

Confirm the scientific name, Trichogaster chuna. A honey gourami is not a dwarf gourami or the larger thick-lipped gourami sometimes sold under similar color names. One avoids the extra complications of keeping a breeding pair in a small community. Provide cover without blocking surface access, and watch feeding to make sure the fish is not intimidated. See the honey gourami care guide for identification and behavior.

Pygmy corydoras: social fish, not cleaners

Pygmy corydoras stay around an inch long and should live with their own kind. They also swim above the bottom, so do not assume they occupy otherwise unused space. Offer suitable small sinking foods directly; leftovers are not a complete feeding plan. A group of eight is a practical choice here, with quiet tankmates and places to rest. Aquarium Co-Op’s pygmy cory guide covers their group size and feeding needs.

Male guppies: a different water-chemistry option

Guppies are a better starting choice when your water is naturally hard and alkaline. An all-male group avoids deliberately adding breeding females, although males can still chase one another. Ask the seller to confirm their sex and have a separation plan for persistent bullying. Do not continually add fish to distract an aggressor. Read the guppy care guide and our guppy temperature guide before setting up.

How many fish can a 20-gallon tank hold?

There is no dependable single number. The inch-per-gallon rule overlooks body mass, activity, territorial behavior, social grouping, and adult growth. Twenty tiny fish and twenty stocky fish place very different demands on the same aquarium.

Use a complete stocking plan, not spare gallons, to decide what belongs. More filtration can help process waste but cannot create territory or swimming space. Stable test results also do not prove that a crowded or incompatible group has enough room.

Choices that need a different plan

Bettas: a 20-gallon aquarium can provide good space for one, but compatibility varies by individual. Do not add one to the gourami plan or assume it will tolerate small fish and shrimp.
Mollies and swordtails: do not substitute them for guppies without checking the exact variety, adult size, activity, and breeding needs. Larger varieties need more room; see our molly guide.
Goldfish and common plecos: leave these out of the small tropical communities above. Buying a juvenile does not solve its eventual housing requirements.
Unspecified “cichlids” or “barbs”: those labels cover very different fish. Identify the species before making any compatibility decision.

Prepare the aquarium before buying fish

Cycle the filter using a fishless method with a controlled ammonia source and testing. Simply running an empty tank for a few weeks does not demonstrate that it can process fish waste. Confirm completion of the chosen cycling method, including ammonia and nitrite returning to zero, before stocking. The RSPCA aquarium environment guide explains why this matters.

Use a suitable heater, thermometer, filter, and secure lid. Add fish in planned stages while preserving each species’ social group, and test after additions. Keep an established alternative available if fish must be separated; an uncycled bowl is not a useful backup.

Keep the plan manageable

Feed measured portions and check that every species eats. Inspect the equipment and fish daily. Use water tests and the tank’s condition to guide regular partial water changes; condition replacement tap water and avoid abrupt temperature changes. Fish, snails, and shrimp do not replace this work.

Give the aquarium a consistent day/night schedule rather than leaving the light on continuously. Adjust lighting to the plants and algae response. If maintenance becomes difficult, address feeding, filtration, and stocking before buying a supposed cleanup fish.

Start with one of the three plans, learn how that group behaves, and allow the aquarium to settle. An attractive 20-gallon tank does not need a specimen of every fish on the store’s small-fish shelf.

Red-Tail Black Shark Care: Tank Size, Diet and Mates

Trevor Aquarium Basics, Aquatic Life (info & tips), Fish Care Guides, Freshwater Aquarium Sharks (Care Guides), Freshwater Fish (info and tips), Tropical Freshwater (guides), Tropical Freshwater Fish (Care Guides) September 15, 2021September 4, 2026aquarium shark, Red Tail Shark, sharks

Red-tail black sharks (Epalzeorhynchos bicolor) are territorial freshwater cyprinids—not sharks. They reach about 5–6 inches (13–15 cm), spend much of their time near the bottom and are best kept as the only red-tail or rainbow shark in a long, mature aquarium. A 55-gallon tank with a four-foot footprint is a sensible minimum starting point for one adult, but layout and tank mates matter as much as volume.

Adult red-tail black shark in a planted freshwater aquarium — A mature red-tail black shark needs a defined territory, visual barriers and open swimming room.

Red-tail black shark care at a glance

Scientific name	Epalzeorhynchos bicolor
Adult length	About 5–6 in (13–15 cm)
Temperature	72–79°F (22–26°C)
pH	Approximately 6.5–7.5; stability matters
Social behavior	Solitary and territorial, especially when mature
Minimum starting point	55 US gal with a four-foot footprint for one adult
Diet	Omnivorous grazer; varied prepared and fresh/frozen foods
Difficulty	Intermediate because of territorial behavior

These figures are practical husbandry guidance, not permission to overcrowd. Upgrade when the fish cannot patrol without continually encountering tank mates.

Species, appearance and conservation

Red-tail black shark showing its black body and red caudal fin — Only the tail is red; rainbow sharks normally show red on several fins.

The red-tail black shark belongs to the carp and minnow family. Its solid dark body and red caudal fin distinguish it from the rainbow shark (E. frenatum), which has red on more fins.

FishBase lists the wild species as Critically Endangered and reports that ornamental exports are now captive bred. That does not make husbandry unimportant: buy clearly identified, captive-bred stock from a responsible seller and never release an aquarium fish.

Tank size and layout

Floor space is more useful than a tall, narrow tank. Start with a four-foot-long aquarium around 55 gallons for one adult and use a larger footprint when keeping active or bottom-oriented companions. Juveniles may appear peaceful in small shop tanks, but territorial behavior generally strengthens with age.

Create several sight breaks with anchored wood, smooth rock and sturdy plants.
Provide at least one roomy retreat without a narrow dead end.
Leave an open route for swimming and feeding.
Use a secure lid; startled fish can jump.
Protect heater glass and filter intakes from impacts and entrapment.

Water conditions and filtration

Maintain a fully cycled aquarium with zero ammonia, zero nitrite and nitrate controlled by stocking, feeding, plants and regular water changes. A stable temperature of roughly 72–79°F and pH around 6.5–7.5 fit the species’ commonly reported range. Avoid chasing a precise pH with repeated chemical adjustment.

Use filtration that provides strong biological capacity and good gas exchange without turning the whole tank into exhausting current. Rinse reusable media in removed aquarium water or conditioned water, and never replace all mature biological media at once.

Substrate and plants

Use smooth sand or rounded fine gravel because this fish spends time grazing and searching near the bottom. Sharp rock can damage the mouth and barbels. Robust plants such as Anubias and Java fern can be attached to wood or rock; ensure decorations cannot collapse when the fish pushes underneath them.

Diet and feeding

Red-tail black shark foraging near aquarium substrate — Red-tail black sharks forage low in the aquarium but are not a substitute for algae control or tank maintenance.

Feed a varied omnivorous diet. A quality sinking or slow-sinking staple can be supplemented with algae-based foods, blanched vegetables and appropriately sized frozen or live invertebrate foods. Do not rely on leftover flakes or expect the fish to live on aquarium algae.

Offer measured portions once or twice daily, watching that the fish receives food without dominating every feeding station. Remove uneaten food and adjust the amount to body condition and water-test results.

Temperament and tank mates

Keep one red-tail black shark per ordinary home aquarium. Do not combine it casually with another red-tail, a rainbow shark, flying foxes or similarly shaped bottom fish. A very large specialist display is not evidence that the same combination is safe in a normal tank.

Potential companions are robust midwater or upper-water species of suitable adult size that share the temperature and water chemistry. Avoid slow, long-finned, timid, tiny or bottom-territorial fish. Compatibility is never guaranteed: have a divider, spare cycled enclosure or rehoming plan ready before problems begin.

Signs the arrangement is failing

Repeated chasing that prevents another fish from feeding or resting
Torn fins, missing scales or fish pinned in corners
Rapid breathing, persistent hiding or loss of appetite
Continuous glass surfing or frantic jumping

Test ammonia, nitrite, nitrate and temperature immediately. Separate injured or relentlessly targeted fish; adding more decorations does not reliably solve an established attack pattern. Seek an aquatic veterinarian when injuries, swelling, ulcers or breathing difficulty persist.

Acclimation and quarantine

Quarantine new fish in a fully cycled, covered system with dedicated equipment. Match temperature, then acclimate according to the transport-water difference without prolonging exposure to dirty bag water. Observe appetite, feces, skin, fins and breathing before introducing the fish to a community.

Breeding

Home-aquarium breeding is uncommon and should not be attempted by placing an adult pair together in a normal tank. Commercial production uses controlled methods and space unavailable to most keepers. Sex cannot be identified reliably from color alone.

Frequently asked questions

Can a red-tail black shark live in a 20- or 30-gallon tank?

Not as a permanent adult. These sizes do not provide enough patrol distance and territorial separation. Plan around at least a four-foot, roughly 55-gallon footprint for one, with more room when the community is busy.

Can it live with a rainbow shark?

Not in a typical home aquarium. Their similar shape and use of the lower tank make conflict likely.

Does it need to live in a school?

No. Unlike bala sharks and roseline sharks, adult red-tail black sharks are normally maintained singly in aquariums.

Will it clean the tank?

No fish replaces maintenance. It may graze surfaces, but it still needs a complete diet, clean substrate, water changes and appropriate filtration.

How long do red-tail black sharks live?

Several years is realistic with good care, but lifespan figures vary and should not be treated as a guarantee. Buy only if you can provide adult housing for the fish’s entire life.

Related guidance and references

Final recommendation

A red-tail black shark can be an impressive single territorial fish for an experienced keeper with a mature four-foot aquarium. Choose it for its natural behavior—not as an algae solution—and avoid it when the planned community contains timid, long-finned or competing bottom-dwelling fish.

Ember Tetra Care: Tank Size, Feeding and Tank Mates

Trevor Aquarium Basics, Aquatic Life (info & tips), Fish Care Guides, Freshwater Fish (info and tips), Tetras (Care Guides), Tropical Freshwater (guides), Tropical Freshwater Fish (Care Guides) September 15, 2021September 29, 2026Ember Tetra, tetras

Ember tetras are small, peaceful fish best kept in a group in an established, planted aquarium. Their orange color looks striking against greenery, but their small size is not a reason to crowd them into a tiny container. Give them swimming space, gentle filtration, and food they can actually swallow.

Ember tetra care at a glance

Requirement	Practical guidance
Scientific name	Hyphessobrycon amandae
Adult size	About 0.8–1 inch
Group	Start with 8–10 of their own species
Tank	10 gallons for a focused group; 20 gallons gives more community options
Temperature	About 76–78°F is a useful everyday target
Water	Soft to moderately hard; slightly acidic to neutral conditions are a useful starting point
Diet	Small complete flakes or pellets, with appropriately sized frozen or live foods for variety

Published ranges are wider than these everyday targets. Fluval’s species profile (PDF) lists 76–82°F, pH 5.5–7.0, and hardness of 1–12 degrees. Aquarium Co-Op describes a broader range for aquarium-kept fish. Ask about your supplier’s water and avoid abrupt adjustments to chase a particular number.

Appearance and behavior

Embers come from Brazil’s Araguaia drainage. They have small, somewhat translucent orange bodies; they are a different species from glowlight tetras. Mature females may look fuller than males, but color and body shape are not foolproof sexing methods.

They often spread into a loose shoal rather than swimming in a tight formation all day. A relaxed group browsing around plants is not necessarily failing to school. Constant hiding, isolation, rapid breathing, or a fish being excluded from food deserves closer attention.

Tank size and group size

Our recommendation is a 10-gallon or larger aquarium for a dedicated group of 8–10. Some care sheets allow smaller tanks, but the extra room makes planting, swimming space, and maintenance easier to balance. Choose a rectangular footprint rather than relying on height alone.

Keep one proper group instead of buying a few individuals from several tetra species. Similar-looking fish do not replace companions of their own species. A larger group also needs a larger overall stocking budget; “small fish” does not mean unlimited numbers.

For a community, see our 20-gallon stocking plans. Those are separate example combinations, not lists to merge.

Build the habitat before adding fish

Complete a fishless cycle and confirm the filter processes the chosen ammonia source, with ammonia and nitrite returning to zero. Merely running a filter in clean water for a set number of weeks is not enough. The RSPCA’s aquarium setup guidance explains why biological filtration must be established first.

Plant around the sides and back while leaving an open swimming area. Wood and suitable plants provide cover; a dark background can make the fish easier to see. Floating plants can shade part of the surface, but leave room for gas exchange and access for feeding.

Use a filter with adjustable or gentle flow and protect exposed intakes. Embers should not have to fight a jet of water continuously. Fit a lid, thermometer, and an appropriately sized heater where needed. Give the aquarium a regular light/dark schedule.

Leaves and wood may tint water, but they are not a reliable substitute for measuring hardness and pH. Do not pour in acidifying products to force soft-water conditions. If your tap water is very hard, consider a fish better suited to it or learn a controlled water-preparation method before buying embers.

Feeding tiny mouths

Offer a small complete food formulated for tropical fish: crush flakes finely or choose truly small pellets. Watch an individual take and swallow food. Repeatedly spitting out large pieces can mean the particle size is wrong, not that the fish needs more food.

Suitable small frozen or live foods can add variety. Offer modest meals once or twice a day and reduce the portion if food reaches the bottom uneaten. A good prepared food is not inherently harmful; the old claim that flakes and pellets necessarily cause digestive problems is unsupported.

Feed away from pushy tankmates and check that the smallest fish gets a share. Aqueon’s tetra guide supports a varied diet and controlled portions.

Choosing tankmates

Small peaceful fish: pygmy corydoras or a carefully chosen honey gourami can be options in a suitably sized community with compatible water.
Shrimp: adult Amano shrimp can be an option, but tiny shrimplets of other species may be eaten.
Bettas: compatibility depends on the individual. Have a suitable separation plan rather than assuming a peaceful outcome.
Larger or boisterous fish: avoid predators and persistent chasers. Even a fish described as peaceful may swallow an ember if it fits in its mouth.

Check the needs of every species separately. Do not add another shoal or a bottom-dweller simply because that area of the aquarium looks empty.

Water changes and signs of trouble

Inspect feeding, swimming, and equipment daily. Test regularly, especially after adding animals or changing filtration. Carry out partial water changes with conditioned, temperature-matched water and remove accumulated debris without replacing all biological filter media at once.

Color alone is not a diagnosis. Newly introduced fish can look pale, but prolonged fading together with weight loss, clamped fins, sores, or unusual breathing calls for investigation. Check water quality first and seek aquatic veterinary advice if illness persists. Our abnormal bottom-resting guide covers initial checks.

Breeding: eggs, not pregnancy

Ember tetras scatter eggs and do not provide parental care. A female holding eggs is not pregnant in the livebearing sense. Adults may eat eggs or fry, so an occasional baby in a planted tank is different from a dependable breeding method.

For a deliberate attempt, prepare a separate mature breeding setup with gentle filtration, protection for the eggs, and very small first foods ready before spawning. Newly swimming fry may need infusoria or appropriately fine fry food before they can take larger items. Raising them requires frequent observation and careful water management.

Do not sharply change pH or temperature to force spawning. Learn routine adult care first; a healthy, settled shoal is already a rewarding aquarium.

Molly Fish Care: Tank Setup and Balloon Molly Advice

Trevor Aquarium Basics, Aquatic Life (info & tips), Fish Care Guides, Mollies (Care Guides), Tropical Freshwater (guides), Tropical Freshwater Fish (Care Guides) September 15, 2021September 29, 2026balloon molly, mollies, Molly Fish

Molly fish do best when their aquarium matches their adult size and preference for mineral-rich water. They are active livebearers, so a successful setup also needs room for social behavior and a plan for unexpected fry.

Balloon mollies are selectively bred, short-bodied forms with rounded bellies. Their shape does not make them suitable for tiny tanks. This guide covers everyday molly care and the extra observations that matter if you already keep balloon mollies.

Molly fish care at a glance

Care point	Practical guidance
Tank size	20 gallons is a starting point for smaller varieties; choose 29–55 gallons or more for larger mollies and communities
Adult size	Varies by species and strain; many common mollies reach 4–5 inches
Temperature	75–80°F, kept stable with a heater when needed
Water	Hard, alkaline freshwater; test your source water and ask about the seller’s conditions
Diet	Complete omnivore food with plant ingredients, plus varied supplementary foods
Mixed-sex group	At least two or three females per male, with enough space and a breeding plan

Aquarium Co-Op’s molly guide explains the differences between common varieties and their housing needs. A small juvenile or a short-bodied balloon strain should not be used to justify crowding.

Choose the tank around the adults

Leave open swimming space and use plants or decorations to break up lines of sight. A secure lid, working filter and thermometer are basic equipment. Position the outlet so water circulates without forcing every fish to swim against a strong jet.

There is no useful rule allowing two mollies per gallon. A proposed group must fit the tank as adults, alongside any other fish and future offspring. For a mixed group, a larger aquarium gives more flexibility than stocking a 20-gallon tank to its limit. Our 20-gallon stocking guide can help compare alternatives before you buy.

Establish the biological filter before adding fish. The RSPCA’s aquarium guidance explains how filtration processes toxic waste and why partial water changes remain necessary. Keep ammonia and nitrite undetectable. Test regularly, remove uneaten food, and use conditioned, temperature-matched replacement water. Adjust maintenance to the tank’s measured waste buildup rather than waiting for cloudy water.

Do mollies need aquarium salt?

Do not automatically add salt to a freshwater molly aquarium. Many mollies can live in hard freshwater. Salt tolerance and a requirement for salt are different things, and other residents or plants may not tolerate an addition.

Check pH, general hardness (GH) and carbonate hardness (KH), not just temperature. Sodium chloride is not a substitute for calcium and magnesium hardness. If your tap water is very soft, discuss a measured remineralization plan with an experienced retailer, or choose fish better matched to it. Avoid trying to correct everything with a sudden chemical adjustment.

Ask whether new fish were held in freshwater or brackish water before purchase. A large difference between the shop and your aquarium needs a planned transition, not guesswork.

What should mollies eat?

Use a complete flake or small pellet appropriate for omnivorous tropical fish, including plant ingredients. Offer modest portions once or twice daily and watch all the fish eat. Occasional suitable frozen foods can add variety; bloodworms or brine shrimp do not need to be the staple diet.

Aqueon’s livebearer care sheet supports a mixed diet rather than a vegetables-only menu. Grazing on algae does not eliminate the need to feed your mollies. Remove uneaten supplements before they decay and reduce portions if food routinely reaches the bottom untouched.

Balloon molly shape and welfare

A balloon molly’s rounded outline is a bred body form, not proof of pregnancy. Do not assume every change in its belly is normal, either. Compare the fish with its own usual appearance and behavior.

When choosing fish, prioritize comfortable swimming, normal feeding and an ability to hold position. Our recommendation is to favor standard-bodied mollies over exaggerated body shapes. The RSPCA’s guidance on extreme conformations explains the broader welfare concern with breeding animals for altered physical features; it is not a diagnosis of any individual molly.

If you already own balloon mollies, give them accessible feeding areas and resting space outside the strongest flow. Persistent floating, sinking, gasping or refusal to eat warrants prompt investigation. Test the water and seek qualified fish-health advice if symptoms continue; body shape alone does not identify the cause.

Social behavior, tank mates and breeding

Males have a narrow, rod-like anal fin called a gonopodium; females have a broader, fan-shaped anal fin. Do not use the dorsal fin alone to sex them. More females can spread courtship pressure, but a numerical ratio cannot fix an undersized tank or relentless harassment.

Choose similarly sized, peaceful companions with matching water needs. Slow, long-finned fish and very small shrimp can be poor choices. Watch interactions after any introduction and have a way to separate incompatible animals.

Mollies give birth to live fry, and adults may eat them. Plan accommodation and responsible rehoming before deliberately breeding. Dense plants offer cover, but they do not create extra stocking capacity. See our guides to molly pregnancy and raising molly fry for the next steps.

A useful weekly check

Confirm temperature, filter operation and water-test results.
Watch each fish swim and eat, rather than checking the group from a distance.
Look for persistent chasing and fish hiding from others.
Count new fry and reassess available space.
Record changes before adding remedies or more livestock.

← Previous