How do fish communicate to each other?

Since this question is in the Betta section I assume you are asking about Bettas reactions to other fish. Bettas are very slow moving and peacefull fish when kept with other species. The only time a male Betta becomes aggressive is if another male Betta comes into his vicinity (Tank). So, if you don't put two Bettas in the same tank, you will never have fighting problems with one by itself or in a community.