Why do Molly fish bite each others butts?

Answer 1

This sounds like part of the 'mating dance' of livebearing fish. Mollies, like other livebearers, practice internal fertilisation - the anal fin of the male is modified into a tube, called a gonopodium, which he uses to place his sperm inside the female's genital opening. Male livebearers live to eat and mate; they harass the females a LOT in order to attempt to mate with them. Following her around incessantly, 'flagging' the gonopodium and nibbling at the female's backside are the ways he will try to get her to stay still for long enough to mate with her. In the absence of enough females, or when there is a receptive female in a small tank and the whole tank reeks of her, males will sometimes attempt to mate with each other. This is normal.

Answer 2

When the other fish have gas they fart and the other mollys play a game like slug a bus only it's called bite a butt.

Hope this helps.