Choose fish that are compatible to ensure your fish can share a tank.
Passive fish do not pose a real threat toward other fishes, and are sometimes reclusive in nature.
These are active fish that might chase other fish or become aggressive towards similar sized tankmates. Check that your current fish are compatible with any new fish that you introduce.
These are territorial fish that should only be housed with other aggressive fish, in a species only aquarium. They can sometimes be housed with semi-aggressive tankmates, but check first.