I think most people stock at one adult fish per 10 gals. of water. A breeding pair could be kept in a 37. Not really enough space or water volume to keep a normal group happy. I think you could keep 6 in a 55 provided you stay on top of water changes. I'm keeping 13 in a 125 gal. with drip, and it's not a problem. However, I plan to take 4 of them out in about a month just to cut down on the load, and to help stock another tank.