Despite what so many otherwise good references say, Chromis sp. are NOT schooling fish. They will tolerate each other when young, but as they mature they become territorial like any other damselfish.
One by one, the less-dominant individuals are killed off by the dominant pair until you're left with two. In smaller aquariums, the weaker of the pair is frequently killed as well. In very large aquaria, you'll end up with several pair, each defending a territory of roughly a square metre in footprint.
It happens faster in the larger species, like the Atlantic blue chromis (C. cyanea), but eventually, all chromis species will become rather a territorial fish.
