The Rainbow Shark (Epalzeorhynchos frenatum) is a species of Southeast Asian freshwater fish from the family Cyprinidae. It is also known as the Ruby Shark, Red Fin Shark, Red Finned Shark, Rainbow Sharkminnow, Green Fringelip Labeo, Whitefin Shark and Whitetail Sharkminnow. It is a popular, semi-aggressive aquarium fish. Unlike true sharks, which belong to the Chondrichthyes «cartilagenous fishes» lineage, the rainbow shark is an actinopterygian «ray-finned fish».
Reportedly native to Thailand, Laos and Cambodia and was once widespread particularly in Thailand where it’s been recorded from the Mae Klong, Chao Phraya and Mekong river basins with occurrences in the provinces of Chiang Mai, Phrae, Phitsanulok, Nakhon Sawan, Chainat, Kanchanaburi, Phra Nakhon Si Ayutthaya, Nakhon Ratchasima and Ubon Ratchathani.
The Rainbow Shark has an elongated black, dark blue or bright blue body. The snout is pointed. The abdominal area is flat. The fins possess red to orange-red colouration. The linear area from the gill cover, the eye, and the mouth has a characteristic brief stripe. Compared to females, male Rainbow Sharks have thinner bodies with black lines along with the tailfins. Males also have brighter colouration. They can grow up to about 6 in (15 cm) long.
Rainbow Sharks are tank-bottom and aquarium-surface cleaners. Being bottom and mid-level dwellers, they consume leftover fish food, but also eat the algae growing off surfaces. They are known to be peaceful with their own kind in the wild but have been known to be aggressive with one another if kept together in a tank. Threat displays and fighting are likely to occur. This fighting behaviour involves head-and-tail butting and also biting. A large Rainbow Shark will continuously chase a smaller one until the smaller one dies, or chase other fish out of its territory, especially in confined environments like aquaria. It may also increase the risk of the fish jumping out of its tank.
Rainbow Sharks are compatible with barbs and Rainbowfish, which are upper and middle tank dwellers. They can also live with danios, loaches, plecos, rasboras, and gouramis. They are not compatible with smaller, more timid fish in the tank, as the sharks may terrorize them by chasing them from their territory.