Red Fin Shark: Size, Tank Mates, Diet and Aggression

A red fin shark is almost always the rainbow shark, Epalzeorhynchos frenatum, a Southeast Asian freshwater fish with a dark body and red to orange-red fins (Wikipedia). Despite the name it is a ray-finned fish in the carp and minnow family, Cyprinidae, not a shark (Wikipedia).

Shops also sell a close relative, the red-tailed shark (Epalzeorhynchos bicolor), which PetSmart listed at $7.99 when checked on 25 September 2026. The two are easy to separate:

• Rainbow shark: red or orange on all the fins, with a black to bright blue body (Wikipedia); FishBase also notes a black blotch on the tail fin
• Red-tailed shark: black body with only the tail bright red (Wikipedia)
• Albino rainbow shark: white body with red or orange fins (Wikipedia)
• GloFish shark: a fluorescent, genetically modified rainbow shark (Wikipedia)

If you are not sure which one you have, a clear side-on photo in the FishLens app will tell you.

About 15 cm (6 in). Both Wikipedia and FishBase give 15 cm as the adult or maximum length. Males tend to be slimmer and more brightly coloured than females (Wikipedia). The red-tailed shark is a similar size: typically 16 cm, with some reaching 18 to 20 cm (Wikipedia).

Yes, as adults, mostly towards fish that look like them. Wikipedia says juveniles are peaceful but adults become highly territorial, and that they do not get along with similarly shaped fish or small, timid species. The red-tailed shark is described the same way: it sets up a territory and chases other fish out of it, with temper varying from fish to fish (Wikipedia). PetSmart rates the red-tailed shark semi-aggressive and non-schooling.

Ways to keep the peace:

• One per tank: keep a single shark, and do not mix rainbow and red-tailed sharks
• Space: a long tank gives the shark a territory without trapping other fish in it
• Hiding places: caves, plants and wood break up lines of sight

Wikipedia lists barbs, rainbowfish, danios, loaches and plecos as suitable companions: active fish that keep to other parts of the tank or are sturdy enough to ignore a chase. Avoid other 'sharks' and fish with a similar body shape, and small, shy species. For the tank itself, Wikipedia gives at least 55 US gallons and FishBase a tank at least 120 cm long; PetSmart gives 50 gallons for the red-tailed shark. A small starter tank is not enough for either.

They are omnivores. In the wild FishBase lists algae, periphyton (the film of algae and microbes on surfaces), phytoplankton and some zooplankton. In the aquarium Wikipedia lists algae, live foods such as tubifex worms, crustaceans, and vegetables such as lettuce and spinach. A mix of sinking pellets or wafers, some frozen or live food and blanched vegetables covers this. PetSmart's feeding note for the red-tailed shark is similar: prepared foods, small live foods and certain vegetables.

The two fish have very different wild stories. The rainbow shark is rated Least Concern (IUCN, via FishBase). The red-tailed shark, endemic to a few river basins in Thailand, is Critically Endangered: it was believed extinct in the wild from 1996 to 2011, and a single fish was found in the Mae Khlong River in 2013 (Wikipedia). The ones sold in shops are captive-bred. It lives 5 to 8 years in captivity (Wikipedia), so either fish is a long commitment.