I know that ocellaris clows stay smaller than percs and I think I would go with them instead. I think most clowns require 30 or bigger but I think ocellaris would be fine in a 20gal tank. In a 20 I would go with no more than 4 small fish such as a firefish and a small goby of sort. But watch alot of your gobies actually get relativly big around 5 or 6 inches full grown. Stay with something on the smaller size dont want to over due fish in the tank.