Rainbow shark

Water parameters

Tolerated range for this species. Aim for the middle of each band rather than the extremes.

Tank and habitat

Substrate: sand.

Behavior

Plant interaction: plant safe.

Feeding

Omnivore that grazes on algae and biofilm but also needs protein. Sinking pellets and algae wafers form the staple diet. Supplement with frozen bloodworm, frozen brine shrimp, and blanched vegetables (zucchini, peas). They eat algae off surfaces but not fast enough to serve as a primary algae control fish. Feed once or twice daily. Sinking food dropped into their territory is claimed and defended. In community tanks, they eat at the bottom after other fish are done with the floating food. Not a picky eater.

Compatibility

• Territorial bottom-dweller that chases fish out of its claimed area. The aggression is mostly about space, not predation. Rainbow sharks rarely cause physical damage to tankmates but the constant chasing stresses smaller or timid fish.
• One per tank is the standard rule. Two rainbow sharks in the same tank fight relentlessly unless the tank is very large (400 L) with separate territories. Odd numbers (3 or 5) don't help; the dominance hierarchy just produces one bully and multiple victims.
• Tankmates should be mid-water or surface-dwelling species that stay out of the shark's bottom territory: barbs, larger tetras, rainbowfish, danios. Avoid other bottom-dwellers of similar body shape (red-tail sharks, Chinese algae eaters) because they trigger the strongest territorial response.
• Aggression increases with age and size. Juvenile rainbow sharks under 6 cm are relatively mild. Adults at 12–15 cm can dominate a 200-liter tank and make life miserable for timid bottom species like corydoras.

Origin and habitat

Epalzeorhynchos frenatum, the rainbow shark, is a bottom-dwelling cyprinid from Southeast Asia, recorded from the Mekong, Chao Phraya, and Mae Klong basins across Thailand, Laos, and Cambodia. Despite the name it is no relation to marine sharks; it is a sharkminnow in the carp family, and the name simply reflects the tall, upright dorsal fin. The body is dark grey and all the fins are a vivid orange-red, with a popular albino form that has a white body and the same red fins. It grows to about 15 cm. It is easily confused with the closely related red-tail shark, E. bicolor, but the rainbow shark has red on every fin while the red-tail has colour only on the tail. In the wild it moves between rivers and floodplains with the seasons, and IUCN lists it as Least Concern, in contrast to the much rarer red-tail. Juveniles are fairly social, but adults turn strongly territorial and solitary, especially toward similar elongated bottom fish like Siamese algae eaters, flying foxes, and Garra.

Breeding

Rainbow sharks are almost never bred in home aquariums. They are egg-scattering cyprinids that spawn in brief aggregations over vegetation with no pair bond and no parental care, but in a tank their mutual intolerance, since two simply cannot share the space, makes natural spawning all but impossible. Sexing is hard, with females perhaps a little fuller and males slightly more intensely coloured. The trade runs entirely on farmed fish produced on Southeast Asian farms, where spawning is induced commercially; tank spawnings at home are rare and mostly unverified.

Common problems

The defining issue is territorial aggression, which intensifies as the fish grows: a small juvenile becomes a fifteen-centimetre enforcer that claims the bottom and chases intruders, and it will fight another rainbow shark or a red-tail shark to the death, so one per tank is the firm rule. Beyond behaviour the fish is hardy. Ich turns up on new arrivals and clears with standard treatment, and faded fin colour, red going to pale pink, is usually a diet issue that improves with better, colour-enhancing food. They are also ready jumpers, startling easily during maintenance, so a tight lid matters.

Bioload

Further reading

• How to cycle a fish tank
• Stocking ideas by tank size