Yes, rainbow sharks are aggressive. However, their behavior is dependent on their environment and tank mates.
Rainbow sharks can become very aggressive and territorial when they don’t have enough space in the tank. Considering their territorial nature, rainbow sharks also don’t tolerate each other or other freshwater sharks well. 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.
However, 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.