When purchasing fish, it’s easy to fall into the trap that fish with generally nice temperaments (or fish of the same species) will play nice together. While this may hold true in nature to some extent, the aquarium presents many challenges. These fish are no longer in an open environment rich with resources. Instead, they are in a small glass box of water without their normal social structure, and they can become quite aggressive toward each other. Fish like chromis will fight with each other to the point that there is only one left. Even clownfish will behave this way too. So, be careful with your purchases and avoid trying to recreate schools of fish in your aquarium.