I tried an anthias school with 4 to start. I don't think this was enough as now I only have one left. I never saw them beating on one another, but as soon as one would die another would start to go downhill. I now have one left that is doing very well, just not the school I wanted. I have been told that I did not start with enough to establish the school.
It all started with one being sectioned off from the others, even appearing to be healthy, then I started noticing marks from getting picked on, etc., etc. - now there is one.