These fish are pretty much schooling cichlids, so really we want to keep groups of them, just like any schooling fish. Between a group of them and the rocks they should have setup in the tank, plus their swimming activity, I really don't think a tank as small as 20 gallons is very good - I would say 40 gallons for a group of at least 6. Tank size recommendations is much opinion though - some people are happy with the bare minimum, while others (like myself) like to provide lots of room for swimming and thriving. It's been a while since I kept them, but I had 15 in a 75 gallon tank and they really are wonderful fish as a proper group with room to actually wander around as a group or spread out depending on their mood.