Yes, rainbow sharks are aggressive. When they don’t have enough space, or when they are kept with the wrong tank mates, rainbow sharks may chase, bite, and headbutt other fish.
Rainbow sharks can become very aggressive and territorial when they don’t have enough space in the tank. If you’re going to keep any other tank mates with a rainbow shark, the fish will need a 75-gallon tank or larger. The bigger their tank, the less aggressive rainbow sharks will be.
Rainbow sharks also don’t tolerate each other or other freshwater sharks. If you keep more than one rainbow shark in a tank, they may fight each other.
Lastly, rainbow sharks are also hostile towards bottom-dwelling fish, such as plecos and catfish. Good tankmates for a rainbow shark would be peaceful top-dwelling fish that are strong enough to defend themselves. Danios, rasboras, barbs, gouramis, and rainbowfish can live with rainbow sharks.
If they are given plenty of space to swim around and are kept with fish that live in the middle and top of the tank, rainbow sharks can behave nicely and be considered semi-aggressive.