It all depends on the species really. Take my lemon tetras for example, there are only 3 of them at the moment and they don't bother any of my fish, they just swim around looking for food in the tank. While my b. neons nip each others fins to determine the leader of them.neville said:yeah I agree, u should add some more tetras. they are schooling fish. at least keep 6 of them. i've heard if kept less than 6 of them, they can become fin nipper!!! is it true???