I've got a Golden Wonder killifish in a 20 gallon, and have removed all the other fish in there except for one pair of platies and a red tailed shark. The killi picks on everybody, although the platies haven't been seriously injured, but you can tell them can never really relax. The red tail has been hiding in a corner for a while (he's just a baby). Basically I'm trying to figure out what can live in the 20 with the killi without getting picked on...?