unless you by some chance have an overly agressive aro or clown there should be no problem .. aro's stay mostly on top as do clown's stay middle to bottom.. expect there to be some peck's at each other during feeding aspecially , and just cruzin the tank.. and tank size is a key factor as well as a cave like object for the knife , knife's like a hiding spot till they get about 8 to 10 '' or so ....