Aggression is one of the most common problems when it comes to keeping pet fish in your aquarium or pond. Injuries caused by aggression can lead to life-threatening bacterial, fungal and parasitic disease in your fish. You can solve this frequent cause of death and illness in an aquarium by fixing incompatibility problems.
The solution usually requires physically separating the aggressive and submissive fish. Many fish hobbyists – especially beginners – want variety in their new aquarium. All too frequently, fish that are attractive (but maintained separately) in the pet store, won’t get along in the same aquarium or outdoor pond.
For example, larger koi will commonly injure or even kill small goldfish. Members of the same species may even kill and eat smaller or weaker members of their own species.
Fish that are scared but manage to survive attacks by a more aggressive fish may go without food if they spend too much time hiding under a rock or in the corner of the aquarium. The responsible pet store clerk will usually advise you on which fish will get along. Despite this advice, some aquarists still gamble and place non-compatible individuals in the same system.