20 gal not sure never tried it. I would suggest tho rather than 1 50% water change weekly make it 2 20% water changes every 3 days to help clear out the urines and any excess nitra..........probably be a good time to try to gett'em on prepared foods rather than feeders, that'll help to also cut down on the nitra.Your gonna have to be super johnny on the spot-disease gets in that 20 and your baby P will be dead before you knew it was even sick. If you can get a larger tank it'll make things much easier on both you and the fish.
It's been my experience that adult P's are easy to keep - it's the young'ens that are the real bugger.