I'de go with an 18" 4"diam... but depends what size other fish are... I try and keep them small enough the other fish can't get caught in them, or large enough to easily swim threw. peacocks can be pretty territorial but in a 180 you may get away with a small group... I would either add 2 more or add none. ideally this keeps the dominant fish from harassing just 1 fish constantly.. it can still happen.. but it makes trying to co-hab most spineys long-term more successful. also if you do this make sure you have a pipe/hide spot for each fish.. plus at least 1 or 2 more extra hideing spots. They can be more territorial then most cichlids, the good thing is at least its just con-specific aggression. expect them to get 12"+ but not all will hit that size, otherwise i'de suggest a larger group.