[You don't absolutly need one for a fish only tank. My friend has a 55 with 2 Hang On filters, and the only fish lost he has ever suffered was from fish eating one another. His water stays in resonable perameters, and his fish look really healthy. With a fish only tank, if you decide not to add a protien skimmer, just let your tank cycle for a long time IMO. And add fish slowly, don't buy 2 or 3 fish at a time. I myself use a protien skimmer, not a very good one (seaclone), but i also have a large refugium area to help that out. The skimmer is just going to make things easier on you.]