It really depends on the temperament of both fishes. I've had my 7" Fahaka with a 10" Red Devil for about 4 years. The devil is unique because he's killed everything else I ever put in there including other devils, plecs, barbs and africans. He bullies and mouths the fahaka to the point of pushing her above the surface of the water, but rarely bites or does damage. I really think it has to do with the fact that the fahaka is female and he devil is male.