I've had bad luck with trying to school chromis (in other words, have more than 4 chromis). I've actually seen very few tanks which have successfully housed a school of green chromis for longer than a year.
Chromis are much more aggressive towards others of the same species than they are with other fish. As you know, these chromis are naturally found in groups of thousands. Within the thousands you may have around 50 dominant specimens "hearding" the group. In the wild, a hierarchy is formed. In our aquariums, this hierarchy needs to be built again. During this time many specimens may be badly hurt.
I recently added a school of 9 Australian green chromis to my aquarium. Over a well later I'm down to 5 green chromis. Many of the larger individuals were very aggressive with eachother for floating food particles. In three of the four death cases, the dominant individual will nip at the fish until it "Goes away" -- this usually means when the fish is an outcast or when the fish is forced into the overflow.
So if i add one or 2 more will this stop the fighting?
You cannot gurentee these fish will stop fighting if you add two more individuals into the aquarium. You may be lucky and end up with 4 happy green chromis, or, you may end up with no chromis after the first month.
Graham
