55g tanks are a real odd size and, while tempting the monster keeper to keep monsters, cannot actually house them in the long run. I've tried many 55g set ups over these last few years and the best so far is a semi-aggressive community. I've got an angelfish, a sparkling jurupari, several catfish and a pair of mollies (the male is pretty lax so it works) and a few random grow outs in there (see my setup for full stocklist. Seems scary but they are all babies) and it's great. Try to stay with fish that don't exceed six to eight inches and are not terrible aggressive. That's what has worked for me at last.