Tetras always work, except if you want to breed. Or the tank is much too small.
That depends a lot on the species and the tank size. E.g. A. hongsloi in a 60 Liter tank are a guarantee for mayhem once the female is brooding and the male will get hacked on a lot. On the other hand, A. nijsseni, which are quite co-operative in brooding but also known to be very aggressive in that situation will terrorize the inhabitants of a 400 Liter tank, but not harm each other.
