Can you just sex them based on their tendency to chest bump? I have three that are a bit smaller than the two largest and they gray out and do their cute little 'I'm the boss' dance for 5 minutes or so. I'd say probably once every two weeks. I just assumed those were males for sure. There are 2 others in there, but they were added after the fact and haven't fully settled in yet. Of course, once they get real big, I guess they would be long past doing that...