Redtail Black Shark Care Guide

Jet-black from snout to tail save for one blood-red caudal fin, the redtail black shark wears the most uncompromising livery of any aquarium 'shark' — and it has the temper to match. Treat it as a territorial loner and keep exactly one to a tank: it attacks its own kind on sight, will not abide a rainbow shark or any look-alike, and reserves its worst for the bottom-dwellers that stray onto its patch of floor. The oddest thing about so common a fish is its back-story — assessed Critically Endangered in the wild, yet shifted by the million, because every specimen in the trade is farm-bred in Thailand. For the record it is no true shark and no loach, but a cyprinid in the carp and minnow family, a close cousin of the rainbow shark.

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 wild life is the whole key to its character: it claims and polices a stretch of riverbed, which is exactly why its hostility falls hardest on other floor-dwellers and why it needs caves and barriers that break the long lines of sight. Its chemistry is forgiving, so what you are really managing is behaviour, not water. The conservation arc is dramatic too — believed extinct in the wild from 1996 until a rediscovery around 2011–2014 in the Mae Klong system, after 1970s dams and pollution gutted its rivers — and yet the entire aquarium trade runs on farm-bred stock.

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 turned up again around 2014 in the Mae Klong system.
• It is no shark and no loach, but a cyprinid — a carp and minnow cousin of the rainbow shark. The 'shark' tag is only about the high dorsal fin and torpedo shape.
• Telling the two black-and-red 'sharks' apart is easy: the redtail is jet-black with only the tail red, while the rainbow shark carries red on every fin — and the pair 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 controlled by stocking, not diluted 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 home base it can defend and lets tankmates slip out of its patrol lane. Aggression climbs sharply as it matures, so floor area and visual cover are the levers that bleed it off. The reported adult size looks contradictory at first — 13 cm in one source, 16 cm in another, the odd 18–20 cm claim — but that spread is mostly total-length versus standard-length bookkeeping, not a real disagreement; plan for a 12–16 cm fish. Prioritise length and floor space over height: this is a substrate fish that lives on and patrols the bottom.

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.

Water parameters in practice

Water chemistry is the easy half of this fish. Aim for about 24–26 °C, a near-neutral pH of 6.5–7.5 inside a wider tolerated 6.0–8.0, and soft-to-moderate hardness around 10–15 dGH. FishBase and Seriously Fish both cap the comfortable temperature near 26 °C, so running it hot buys you nothing. It is hardy and adaptable on numbers; what it actually needs is a fully cycled, stable, well-filtered tank. Spend your effort on the social rules — get those right and the chemistry quietly looks after itself.

Diet & feeding

An aufwuchs grazer by trade: in the wild it rasps algae, small crustaceans and insect larvae off rock, rounding the diet out with chironomids, Daphnia, Artemia and plant matter. In the tank, feed it as an omnivore with a genuine vegetable streak — quality sinking pellets and flake carrying a strong algae and vegetable component (algae wafers, spirulina, blanched greens), topped up with live or frozen bloodworm, brine shrimp and tubifex. Offer food once or twice a day; it forages along the bottom all day and will graze nuisance algae as a side effect. The greens are not optional — strip them out and an all-protein diet leaves this rock-grazer short.

Gear & setup

Run a heater into the mid-20s Celsius and a moderate current, in keeping with its clear-water river origin, over sand or fine gravel that suits its rasping mouth. Hardscape here is not décor but behavioural engineering: caves, rockwork, bogwood and dense planting carve the floor into a defendable home patch and chop up the long sightlines it uses to lock onto a target — and that, more than tank size alone, is what keeps a tankmate alive. Fit a tight lid, too — Aquadiction calls securing the tank against escape imperative.

Temperament & behaviour

Solitary, territorial and aggressive — that is the whole story. Wikipedia records it as territorial and hostile to other fish, especially intolerant of other red-tailed black sharks, and notes it will attack members of its own species; Seriously Fish adds that it grows more territorial with age, shows particularly high aggression toward similar-looking species, and is best kept singly in most cases. Juveniles can be shy, but the temper hardens fast with maturity and in a cramped tank. Its fury is reserved for fish that share its floor zone, and there is no shoaling safety-in-numbers to soften it. Give it a single specimen, a cave-broken layout and quick, dissimilar company that holds the upper water — robust schooling cyprinids such as tiger barbs are the classic choice — and it becomes genuinely manageable.

Group & social needs

Solitary — keep exactly one per tank. Two redtail black sharks, or a redtail beside a rainbow shark or any similar-shaped fish, will fight, often until one is dead or permanently in hiding. This is not a shoaling species and must never be grouped; here, more is strictly worse. Build its companions instead from fast, dissimilar mid- and upper-water fish that keep clear of the floor. Active schoolers such as zebra danios suit the role, holding the top of the tank while the shark owns the bottom. Sexing is a dead end: Seriously Fish notes young specimens cannot be told apart externally with any accuracy. The keep-one rule is the single headline fact of the species.

Compatible tank mates (preview)

A short, engine-cleared shortlist — the species TankStocking's welfare engine clears with Redtail Black Shark and that suit its size and temperament best. Tap a name for its care guide, or use + to load the pairing in the planner.

• Convict Cichlid+Similar adult size
• Golden Wonder Killifish (Striped Panchax)+Uses the top/surface zone, similar adult size
• Jewel Cichlid+Similar adult size
• Paradise Fish+Similar adult size

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 raised on commercial farms through the artificial use of hormones. This is what resolves the conservation paradox: the wild fish is Critically Endangered, yet FishBase notes tens of thousands are exported from Thailand each year, all now captive-bred — Wikipedia reports the country shipped as many as five million a year in 1987. Adult territoriality makes holding a compatible pair at home unrealistic, so treat home breeding as a non-starter.

Lifespan

Five to eight years is typical, with good care pushing toward a ceiling near ten. What cuts it short is rarely disease: chronic stress in a too-small tank, wounds from a second shark or a congener, running battles with harassed bottom-dwellers, and poor water quality. Keep one fish, give it space and cover, pair it with the right company, and you have a hardy, long-lived 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. The matched shape — and, with the rainbow shark, the shared 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 all share its floor territory and get driven off it — which is why those species are excluded from this fish's tankmates here, even when a coarse size match would otherwise 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, usually 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 hardening temper 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

Sources & confidence

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 →