While Chromis' are always seen as a schooling fish, there is definitely a pecking order...... What will happen is that one fish will always be outcast, and it will end up dead......until there is only one. They don't beat up on the outcast fish, the outcast typically hides in a corner and starves itself. I started with 10 in my 180. I am down to 4. The runt of the remaining 4 is noticeably less active than the other 3.......won't be long.
So when you get down to your last one, avoid buying more.