Which part? Many damsels, especially Chrysiptera, don't shoal at all. They live in loose, spaced aggregations where each has a decent territory--usually the size of an entire tank. That is why they are aggressive, because they have defined territory. Chromis don't really define territories so much, but they have a heavy hierarchy, as do most shoaling fish. Chromis and similar fish will appreciate other fish out in the open, as it lets them know there is no danger in the open area. These other fish are known as "dither" fish. Normally, other Chromis would play this role.