Monitor aggression and re-aquascape the aquarium, if necessary. If you are getting aggression in your aquarium, the best thing to do is rearrange everything in the tank to disorient the fish. This makes them look for their territory and can reset the hierarchy of the fish again. Adding more fish often helps reduce aggression too. It is important to have a good boss in the tank. A good boss will keep the other fish in check by pushing them all around to display dominance without hurting any. A bad boss will attack and kill the fish one by one.

Many fish are more aggressive if there is not many fish in the tank. Higher volumes of fish can allow fish to feel more relaxed and less stressed.

As per.com Paul Talbot
Taken from www.cichlidhub.com