My chromis schooled for a month. Then they systematically killed each other off one by one. In nature, this is probably a very good survival methodology to ensure only the strongest live to reproduce. I'm going to wager a super wild guess that their reproductive rates in nature are just slightly higher than their kill-off rate. But, when you take a few of them, put them in an environment where they won't reproduce, you'll eventually end up with one or two. I've also heard some anecdotes about large schools surviving longer because aggression is spread out a bit, but who knows.
Moral of the story, chromis school when scared, kill eachother when comfortable.. They are so cute though, who knew..
jb