I have notied they are defensive.. I had him in with a pair of breeding kribs, and they would dual all the time.. However, at the same time there were a number of tetras and dwarf cichlids in the tank, and he ignored anyone who wasn't a threat to him.. I would imagine competition for food would make them more aggressive as well. They can be territorial, but I don't know if they would waste their time chasing mollies and other peaceful tankmates.. I think if they were king of the tank with little threat, they would be very passive.