all depends on the fish and its temperment, i had a synspilum that killed every tank mate he had(jag,midas,oscar,even two pirhanna)all bigger than him. Had to rehome him to keep from losing fish. so it all depends there are always exceptions to the rules. I had a 13 inch dovii that would never show aggression to anything and lived with other fish peacefully. There is no 100% meanest cichlid. If you ask people what is the meanest cichlid they ever owned you might be surprised at all the different answers. btw i actually put him in with the pirhanna to get rid off him and it didnt work, lol