Despite its formidable moniker, the rainbow shark, Epalzeorhynchus frenatum, is a small freshwater fish found in many home aquariums. Also called the red-finned or ruby shark, this is a sleek and slender fish that measures 6 inches at maturity. They are territorial, which makes them somewhat aggressive. A large tank, 20 gallons or more, is the best habitat for a community that includes a rainbow shark.
Choose wisely when selecting tank mates for your shark, or you will find you are constantly adding new fish. The stress of living with a shark could cause a fish to leap out of the tank, die of exhaustion from being chased and harassed or succumb to disease because the stress compromised his immune system. Any fish smaller than a rainbow shark will be at risk; fish the same size or larger than a rainbow shark may fare better, but it's not guaranteed. Some rainbow sharks will challenge a larger fish and only back down when its clear he can't win. It's also best to have only one rainbow shark at a time, as they generally don't get along with their own kind.
Most species of gouramis, with the exception of the tiny dwarf gourami, are good tank mates for rainbow sharks because at maturity they are about the same size as rainbow sharks. Gouramis are labyrinth fish, which means they sometimes come to the surface to breath oxygen. So they tend to swim about the middle to the top of the tank, which suits the bottom-dwelling rainbow shark just fine. For greater success, be sure the tank is full of plants, caves and decor that provides plenty of hiding places. Sharks like to hang out in a secluded spot, and the other fish know to stay away.
All species of pleacos, also spelled pleco, are also good tank mates for the rainbow shark. These are the bottom-feeding catfish-type fish you see sucking on the bottom and sides of the tank. They keep to themselves and pose no threat to the shark. Some species of pleco grow to be 12 inches or more, so be prepared to transfer your pleco to a pond if your tank cannot accommodate a foot-long fish. If you have more than one pleco, be sure to purchase different types of plecos. Two common plecos will not tolerate one another, but a common pleco and a bristlenose catfish, for example, will do fine.
All loaches are suitable tank mates for your rainbow shark except the dwarf loach. Although they live at the bottom like the shark, loaches present no threat to the shark due to their size. But that's not the only reason loaches are good in a community tank with sharks. Most species are shy, reluctant fish that tend to stay inside any hiding places in the tank. So provide them with low caves using ceramic half-logs or other decor that will offer them a cave. Clown loaches are good with rainbow sharks, but require at least a 75 gallon tank. Kuhli loaches can be kept with rainbow sharks, but should be in a group of at least three kuhlis.