Depends on the size of the tank and the species, but the males can be extremely aggressive to females and especially other gouramis. Females can be aggressive to other females as well. And a male will go into murder mode if he makes the bubble nest and breeds. Generally gourami don't mind smaller fish, but they'll probably eat any eggs left on the tank glass. If you keep more than one gourami at a time, I'd suggest a forest of plants for hiding places.