How do you control agressive fish in a social tank?

Social fish may not hold territories but they do jockey for position in the group, and a certain amount of chasing and aggressive display is normal. But if the group is sufficiently large and contains a good balance of males and females, schooling fish shouldn't cause one another serious harm.

Cichlid fish are the classic examples of territorial fish. Territories may be used to control access to food, to impress females, or as safe places to rear young. Problems occur when there isn't enough space for both territory holders and the other fish in the community. Unable to leave the aggressive fish's territory, the other fish end up being harassed, damaged, even killed.