I have successfully kept 9 chromis in a 60 gallon cube for close to a year. Perhaps at the very beginning a couple small ones were excluded from the school, they would just hide in a corner and not compete very much for food. They eventually perished. I sold them last week to someone with a 210 gallon because mine were getting quite large. I didn't really see a lot of issues with aggression. There was an occasional short chase, or sometimes two males might do their weird fight trying to court a female. It just looks like they are open mouth kissing each other. Not sure for the exact reason for my success. A few thoughts: I bought them all as very small juveniles, that may have helped. I also had a very densely stocked tank. With rock, coral and fish. There were plenty of hiding spaces and territories, each had its own. With numerous other fish in the tank, it may be they were distracted enough not to fixate on anyone in particular. When I would add a new acro colony it was pretty fascinating, as they would all school and hover in and around it. I know they use them for protection, but I can't really explain that behavior. Maybe checking it out to see if another fish would come out of nowhere to chase them away. Most aggression I saw was if a new fish tried to take refuge in their burrow while they were out and about. But that was only aggressive tail fin wagging to try and usher them out. I also kept then very well fed. I was initially drawn to them because they are dither fish. Other fish look to them as the all clear signal. If they are out, everything is good, and other fish feel more comfortable being out. Every now and then the school would get spooked and shoot down into their holes, it caused a simultaneous explosion of every single fish in the aquarium simultaneously darting downwards. I will be doing a school of juvenile chromis once again once my new 93 cube is finished cycling.