I haven't heard of anyone successfully keeping emperor cichlids (Boulengerochromis microlepis) to breeding size (18") in anything under 900 gallons. Males are highly territorial and, most likely, only one male will be left in a group. Mine killed all the other males at around 14". Dominant males have also been known to kill all tankmates including harem females for no particular reason. A monster tank is definitely needed for these cichlids since they can reach 3 feet.