Rainbow sharks are more closely related to goldfish than to true sharks. Still, they are handsome fish who superficially resemble their toothy saltwater namesakes. Their aggression makes selection of tank mates complicated but not impossible.
While the rainbow shark (Eplazeorhynchos frenatum) has a milder temperament than some other freshwater fished with "shark" in their names, the rainbow may still have trouble getting along with bottom-dwelling fish. However, the rainbow shark tends to get along with loaches, bottom-dwellers from its native range. However, individual rainbow sharks do vary in their temperament and age. Some individuals may consider any interloper in their territory at the bottom of the tank fair game.
A rainbow shark will almost universally get along fin with peaceful, mid-water schooling fish, such as rasboras and danios from their native range, and tetras from Africa and South America. The smallest rasboras and tetras may be small enough for a rainbow shark to make a meal of them. However, only exceptionally small species will fit into a rainbow shark's little mouth.
With most freshwater "sharks," you can only keep one per tank. While the rainbow shark is less territorial than most of its relatives, you can still only keep one per tank under most circumstances. Even males and females will usually fight among themselves in the confines of an aquarium. If you really want to keep more than one in an aquarium, your best bet is to get a huge aquarium, larger than 75 gallons. Additionally, you should include lots of rocks and driftwood. This gives the sharks places to hide from these other and "breaks up sightlines." When the sharks can't see each other, they get less aggressive.
What to Avoid
A rainbow shark needs to avoid several fish. Other than loaches, rainbow sharks will pick on most bottom-dwelling fish like cichlids and catfish. You should also avoid long-finned fish, like freshwater angelfish, since the rainbow shark may nip at long-finned fish. At the same time, small, nippy fish like rasboras and certain tetras may nip at the rainbow shark. The rainbow shark's brightly colored fins make it a tempting target for fish with this proclivity. Above all, avoid other aquarium "sharks" like the black shark and the red-tailed shark. These species resemble each other enough to trigger their territorial response.