Boxa
Fish Fanatic
Depends how big they are and what temperament that have, I wouldn't really like to say as I have had all different amounts in those size tanks "2 - 4 and up to 6, and others would probably say how dare you 5cups of water per fin blah blah.. 20G isn't ideal for a large number don't forget they are very nasty fish even more so when breeding and they do get biggish.