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

Native to Southeast Asia, documented in the Mae Klong, Chao Phraya, and Mekong river basins across Thailand, Cambodia, Laos, and Vietnam; also reported from Malaysia and Myanmar. A cyprinid (carp/minnow family), not a true shark; the common name comes from the tall dorsal fin silhouette resembling a marine shark. Inhabits rivers and flooded plains, exhibiting seasonal migratory behavior between flowing river habitats and floodplain areas. Gray body with vivid orange-red coloration on all fins; an albino variant (white body, red fins) is also widely available. Grows to about 15 cm. Can be distinguished from the closely related red-tail shark (E. bicolor) because the rainbow shark has red on all fins, while the red-tail has red only on the caudal fin. Juveniles are somewhat gregarious, but adults become increasingly territorial and solitary, particularly aggressive toward similar-looking species such as Siamese algae eaters, flying foxes, and Garra. Best kept as a single specimen per aquarium; will not tolerate another rainbow shark or a red-tail shark in the same tank. Lifespan reported at 5-8 years typically, with some records of up to 15 years.

Breeding

Not bred in home aquariums under normal conditions. Commercial breeding uses hormone injection to induce spawning. The exact protocols are proprietary to Southeast Asian fish farms. Sexing is difficult; females may be slightly plumper when gravid, and males may show slightly more intense coloring, but these differences are subtle. Without hormones, the fish don't spawn in tanks. Occasional reports of spontaneous spawning exist but they're extremely rare and usually unverified. Every rainbow shark in stores is farm-bred.

Common problems

Territorial aggression is the main problem (covered above). Beyond behavior, rainbow sharks are hardy. Ich is common in new purchases; standard treatment works. They're tolerant of a wide parameter range. The main tank-related issue is jumping: rainbow sharks jump when startled, and they startle easily during water changes and tank maintenance. A tight-fitting lid is necessary. White spot disease (not ich; a different condition causing white raised lumps) occasionally appears and responds to antiparasitic treatment. Dietary deficiency shows as fading fin color; red fins turn pale orange or pink. This reverses with a better diet including spirulina and color-enhancing food. The most common "problem" is behavioral: the keeper buys a cute little 5 cm shark for a community tank and discovers 6 months later that they own a 12 cm territorial enforcer that has rearranged the tank's social order.

Bioload

Further reading

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