Rainbow shark

Quick facts

Adult size

13 cm

Lifespan

can live up to 8 years

Tank zone

bottom

Temperament

semi-aggressive

Difficulty

beginner

Water parameters

Temperature

24–27°C

pH

6.5 to 7.5

Hardness

5 to 15 dGH

Tank requirements

Minimum volume

200 L

Minimum length

100 cm

Flow

moderate

Lighting

any

Substrate

sand

Driftwood

preferred

Hiding spots

needed

Open swimming room

needed

Lid

required - jumper

Feeding

Diet: omnivore, feeds primarily at the bottom.

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.

Vegetable matter required (algae wafers, blanched zucchini, spinach).

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.

Habitat

Native to river basins in mainland Southeast Asia: the Mekong, Chao Phraya, and Xe Bangfai in Thailand, Laos, and Cambodia. Found over sandy and rocky substrates in flowing rivers. Wild populations have declined due to habitat modification (dam construction, channelization), and the species is listed as Endangered on the IUCN Red List. Despite this conservation status, the aquarium trade has no impact because all specimens are commercially bred in massive numbers across Southeast Asia. The fish has been in the hobby since the 1960s. The body is dark gray to black with bright red-orange fins (all fins, not just the tail; the red-tail shark has red only on the caudal fin). An albino variant with a pink body and red fins is commonly available. Adults reach 12–15 cm, which is larger than most keepers expect when buying a 5 cm juvenile. The "shark" common name refers to the dorsal fin shape, not any relationship to actual sharks.

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

Bioload coefficient: 3.5 (medium-bodied bottom-dweller; moderate waste).

Bioload coefficients are calibrated against the neon tetra as the anchor (1.0). See the methodology page for the formula and how each value was derived.

Further reading

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