Firemouth are unpredictable like most New World cichlids, from my experience they are very territorial and aggressive. Luckily they do not have the power to do any real damage just mainly tear up fins, they are not afraid to attack a much larger fish. Mine tolerate certain fish and absolutly hate others. Be careful that they do not stress out fish that they pick on because they seem to claim a large territory and can keep another fish pinned in a small part of the tank. Mine seems to claim the middle of the tank so the fish he does not like are stuck on either side but luckily it is a 6 ft tank and they have plenty of room. This is only my experience but im sure others have different opinions.