Michael Pauers of the Medical College of Wisconsin found that male cichlids have no time for other males that look like them. They will bite, butt and threaten those who bear the same colour scheme.

Notably, in the social system of cichlid fish, the least aggressive individuals in the group can often end up wielding the greatest influence.

Aggression and welfare in a common aquarium fish, the Midas cichlid. Journal of Applied Animal Welfare Science 14(4), 340-360. Many species of fishes are aggressive when placed in small aquaria.