I had a group of 10 in my 265 a few years ago and they started to pick each other apart, They were in there for a few years before they did this but it still happened, as they got larger and I mean the largest chromis you can imagine they started going crazy. They will kill each other off just wait. While my tank wasn't the size of a 450 it was larger than the 220 mentioned.Chromis don't kill each other off, I've had my school of 9 for over 8 months now and every one of them are still in the tank, I count them every day lol