Please believe that if you put fish into a too-small tank, you get aggression. There are some species that will eat each other. That's lunch, not aggression. But if fish are fighting, the remaining issues are mating, or the tank is too small for the fish that are in there. There are a handful of fish that are just nippy on anything in reach---the 6-line wrasse and the dottyback;...but for most, give them enough room and they have better things to do than chew up the neighbors.