Redtail Black Shark Care Guide

The redtail black shark is a striking jet-black fish with a single blood-red tail — and an aggressive, territorial loner. Keep exactly one per tank: it attacks its own kind and is lethally intolerant of other 'sharks' and similar-looking fish, and it is worst toward the bottom-dwellers that enter its floor territory. It is neither a shark nor a loach but a cyprinid, the same genus as the rainbow shark, and remarkably it is Critically Endangered in the wild even as it is sold by the million from Thai farms.

Redtail Black Shark at a glance

The sourced figures the welfare engine uses to judge Redtail Black Shark — the parseable key facts.

Where it comes from

Epalzeorhynchos bicolor was described by Hugh M. Smith in 1931 and is endemic to Thailand, historically the lower Mae Klong, Chao Phraya and Bangpakong rivers. Seriously Fish describes its biotope as a flowing stretch of clear water over a metre deep, with a substrate of sand and large rocks — a bottom-oriented river fish. That life explains its temperament: it claims and defends a floor territory, which is why its hostility falls hardest on other bottom-dwellers and why it needs caves and broken sightlines. Its chemistry is adaptable, so the constraint is behaviour, not water parameters. The species has a dramatic conservation story too — believed extinct in the wild from 1996 until a rediscovery around 2011–2014 in the Mae Klong system, after 1970s habitat destruction — yet the entire trade is farm-bred.

Did you know?

• It is Critically Endangered in the wild — assessed CR in 2011 — yet sold by the million. The paradox resolves because the entire trade is farm-bred in Thailand, which exported as many as five million a year in 1987.
• It was thought extinct in the wild from 1996 to 2011, then a small wild population was rediscovered around 2014 in the Mae Klong system.
• It is not a shark and not a loach — it's a cyprinid, a carp and minnow relative, the same genus as the rainbow shark. The 'shark' name is only about the high dorsal fin and shape.
• Telling it from the rainbow shark is simple: the redtail is jet-black with only the tail red, while the rainbow shark has red on all its fins — and the two can never share a tank.
• The epithet bicolor means 'two-coloured', for the jet-black body and the single red tail. It is a textbook case of aggression managed by stocking, not by numbers.

Tank size — and why

A 55 US gallon, four-foot tank is the welfare-safe floor; the 30-gallon figure some sources quote is too small for an adult. The driver is territory, not bioload — a longer footprint gives the fish a defendable home base and lets tankmates escape its patrol zone. Aggression rises sharply as the fish matures, so floor space and broken sightlines are the main tools for diffusing it. Prioritise floor area and length over height: this is a bottom-oriented territorial fish, around 12–15 cm and occasionally to 16 cm, that lives on and patrols the substrate.

Keep a single Redtail Black Shark — its own kind fight, so the answer is one regardless of tank size, with non-rival tankmates added only in a larger, planted tank.

How big does it really get?

Full-grown Redtail Black Shark reach about 15 cm (5.9 in) long, but they are usually sold at only about 3 cm (1.2 in) — a typical shop size (estimate). At full size, Redtail Black Shark needs roughly a 55-gallon tank, about 122 cm long; a common 10-gallon starter kit is only about 51 cm.

Adult size is sourced; the shop size is a typical-juvenile estimate; tank length is approximate for a standard 55-gallon aquarium.

Water parameters in practice

Chemistry is the easy part. Aim for about 24–26 °C, a near-neutral pH around 6.5–7.5 within a wider tolerated 6.0–8.0, and soft-to-moderate hardness near 10–15 dGH. FishBase and Seriously Fish cap the comfortable temperature at roughly 26 °C, so there is no benefit in running it hot. The species is hardy and adaptable on parameters; it wants a fully cycled, stable, well-filtered tank. The real difficulty with this fish is temperament, not water — get the social rules right and the chemistry largely looks after itself.

Diet & feeding

In the wild it is an omnivore and aufwuchs grazer, rasping algae, small crustaceans and insect larvae off rocks, supplemented with chironomids, Daphnia, Artemia and vegetable matter. In the tank treat it as an omnivore with a real vegetable streak: quality sinking pellets and flake with a strong algae and vegetable component — algae wafers, spirulina, blanched vegetables — supplemented with live or frozen bloodworm, brine shrimp and tubifex. Feed once or twice a day; it grazes throughout the day and will help control nuisance algae. Make sure the greens are genuinely in the diet.

Gear & setup

Run a heater into the mid-20s Celsius and moderate flow, in keeping with its clear-water river origin, over sand or fine gravel that suits its grazing mouth. Hardscape is not decoration here: caves, rockwork, driftwood and dense planting to create territories and break up sightlines are the single most important tool for keeping any tankmates safe, because a defendable home base and broken lines of sight genuinely reduce the aggression. A tight, well-fitted lid is also needed — Aquadiction calls securing the tank against escape imperative.

Temperament & behaviour

Solitary, territorial and aggressive — the defining trait. Wikipedia records that it is territorial and aggressive to other fish, especially intolerant of other red-tailed black sharks, and will attack members of its own species; Seriously Fish notes it becomes increasingly territorial as it grows and shows particularly high aggression toward similar-looking species, recommending it be kept singly in most cases. Juveniles can be shy, but aggression rises sharply with maturity and in cramped tanks. Its hostility is worst toward fish that share its floor zone — and unlike a shoaling fish, there is no safety in numbers here. With one shark, plenty of caves and fast mid- and upper-water company, it is manageable.

Group & social needs

Solitary — keep exactly one per tank. Two redtail black sharks, or a redtail with a rainbow shark or any similar-shaped fish, will fight, often until one is dead or permanently hiding. This is not a shoaling fish and must never be grouped; more is worse, not better. Sexing is difficult — Seriously Fish notes it is effectively impossible to sex young specimens accurately by external means. The keep-one rule is the headline husbandry fact of the species.

Compatible tank mates (preview)

The engine clears no fish into a clear top set with Redtail Black Shark. It is not a species you can stock from a generic "peaceful community" list — shrimp, snails and small community fish are not safe defaults with it, so work from the temperament and tank-mate guidance in the sections above (and the full compatibility checker) rather than a quick shortlist.

This engine-cleared shortlist is Redtail Black Shark's tankmate surface for now — a dedicated tank-mates guide can follow for high-demand species.

Breeding & sexing

Not bred in the home aquarium. Seriously Fish records that it has not been bred in private aquaria and that large numbers are produced on commercial farms via the artificial use of hormones. The conservation paradox is resolved by this: the wild fish is Critically Endangered, yet FishBase notes tens of thousands are exported annually from Thailand, all now captive-bred — Wikipedia reports Thailand exported as many as five million a year in 1987. Adult territoriality makes holding a compatible pair at home unrealistic, so this is not a project to attempt.

Lifespan

Around five to eight years in the tank, with good care reaching a ceiling near ten. What shortens it is rarely disease: chronic stress in a too-small tank, injuries from being kept with a second shark or a congener, fights with harassed bottom-dwellers, and poor water quality. Keep one fish, give it space and broken sightlines, and the right company, and it is a long-lived, hardy centrepiece.

Common mistakes

• Keeping more than one — the fatal mistake. Two will fight until one is dead or permanently hiding; it attacks members of its own species. One per tank, full stop.
• Adding a rainbow shark or flying fox. Same shape plus, with the rainbow shark, the same genus triggers lethal aggression — which is why the rainbow shark is excluded from this fish's recommended tankmates here.
• Pairing it with bottom-dwellers. Corydoras, plecos, otocinclus, loaches, Siamese algae eaters and bottom-feeding catfish or cichlids share its floor territory and get harassed off it — which is why those species are excluded from this fish's tankmates here, even when a coarse size match would clear them.
• Buying it for a peaceful community tank. It is territorial and aggressive, not a team player.
• Too small a tank. It needs 55 gallons / 4 ft+ with caves and broken sightlines; the 30-gallon floor some sources cite is too small for an adult.
• Thinking it's a shark or a loach. It's a cyprinid — a carp and minnow relative, the same genus as the rainbow shark.

Signs of trouble

• Faded or greying black colour and clamped fins — chronic stress, often from a cramped tank, fighting, or poor water.
• Visible bite or fight damage — almost always a second shark, a rainbow shark, or a harassed bottom-dweller in the tank.
• A tankmate constantly chased off the substrate or hiding away — the redtail is enforcing its floor territory against a fish that shouldn't be there.
• Refusing food in a normally keen grazer — investigate water quality and stress.
• White spots with flicking and scratching — ich, usually secondary to stress, chilling or poor water rather than a signature disease.

Is this fish right for you?

Don't buy a redtail black shark if you want more than one 'shark', if you keep corydoras, plecos, otocinclus, loaches or other bottom-feeders, if your tank is small or short, or if you wanted a peaceful community centrepiece — its aggression with age and the keep-one rule make it a poor team player. On stock and ethics: all trade stock is farm-bred (the wild fish is Critically Endangered), so buying captive-bred is fine; pick an active, deep-black fish with an intact red tail and no fight damage, and avoid any dyed or 'painted' specimens.

Common questions

Can I keep two redtail black sharks together?

No. Keep exactly one per tank. Two will fight, often until one is dead or permanently hiding — the species will attack members of its own species. There is no safety in numbers here; more is worse.

Can a redtail black shark live with a rainbow shark?

No. They are the same genus and the same body shape, which triggers lethal aggression. Never house a redtail with a rainbow shark, a flying fox, or any other similar-looking 'shark'.

Can a redtail black shark live with corydoras, plecos or loaches?

No. As a floor-territory holder it is worst toward bottom-dwellers — corydoras, plecos, otocinclus, loaches and Siamese algae eaters share its zone and get harassed off it. Good tankmates are fast, confident mid- and upper-water fish that stay out of its territory.

What size tank does a redtail black shark need?

55 US gallons, around four feet, for a single adult, with caves and broken sightlines. The driver is territory, not bioload, so prioritise floor area and length. The 30-gallon figure some sources cite is too small for an adult.

Is a redtail black shark a real shark?

No — it's a cyprinid, a carp and minnow relative, the same genus as the rainbow shark. It is not a loach either, despite sometimes being grouped with them. The 'shark' name is purely about the high dorsal fin and torpedo shape.

What's the difference between a redtail black shark and a rainbow shark?

Look at the fins. The redtail black shark is jet-black with only the tail red; the rainbow shark has red or orange on all its fins. They're the same genus and shape, so they must never share a tank.

Sources & confidence

Sources & confidence (1 species)

These back the Redtail Black Shark figures and the previewed tank mates above. Each figure is read from the TankStocking species database (v2026.06); below is the care reference behind it and how confident we are in that data. Confidence reflects the source quality, not whether any pairing is safe. Full source list and the welfare model are on the methodology page.

• Redtail Black Shark Epalzeorhynchos bicolor — Seriously Fish — Epalzeorhynchos bicolor (https://www.seriouslyfish.com/species/epalzeorhynchos-bicolor/) high confidence

Care-guide sources (5)

This guide synthesises the references below; where they disagree, the range and the disagreement are noted in the text above. The figures in the key-facts box are read from the TankStocking species database (v2026.06). Full welfare model on the methodology page.

• FishBase — Epalzeorhynchos bicolor — authority (Smith 1931), family Cyprinidae, 13.0 cm TL, temp 22–26 C, pH 6.5–7.5, dH <=15, range (Chao Phraya/Mekong, Thailand), IUCN Critically Endangered (23 Feb 2011), omnivore diet, 'tens of thousands exported annually... all now captive bred'
• Seriously Fish — Epalzeorhynchos bicolor — synonym Labeo bicolor, type locality, clear-water rock/sand biotope, SL 100–125 mm, footprint 120x45 cm, temp 20–26 C, pH 6.0–8.0, 36–268 ppm, 'kept singly', 'aggression towards similar-looking species', 'bottom-dwelling fishes including cichlids and most catfish are best avoided', aufwuchs grazer, sexing impossible, hormone breeding, CR status / 2014 rediscovery
• Wikipedia — Redtail shark — Smith 1931, Cyprinidae, 16 cm (rare 18–20 cm), 5–8 yr lifespan, 'will attack members of their own species', keep one, CR, extinct-in-wild 1996–2011 + rediscovery, all captive-bred (Thailand exported up to 5 million/yr in 1987), neutral pH, 22–26 C
• Aquadiction — Red Tailed Black Shark — max 16 cm, up to 10 yr, aggressive/territorial, avoid Rainbow shark / True Flying Fox, temp 22–26 C, pH 6.5–7.5, 10–15 GH, 'well-fitted lid is imperative', omnivore, 'not a true shark' (Cyprinidae)
• Aquarium Source — Red Tail Shark 101 — 4–6 in, 5–8 yr, 55 gal minimum, very territorial, avoid bottom-dwellers + similar-looking sharks, temp 72–79 F, pH 6.5–7.5, omnivore, caves/decor for broken sightlines

More on Redtail Black Shark

Related guides on TankStocking — each scored by the same welfare engine as the planner.

This care guide is a sourced planning reference, not veterinary advice — individual fish, filtration and maintenance all matter. Cycle the tank, test your water, and observe your fish. How TankStocking works →