I have a 10'' tiger oscar and a 7" albino that used to be in a 55gal. Were smaller then. I now have them in a 120gallon. When I was running my 55 I had a 100 gallons of of filtration. NOw I have 190 of filtration in my 120. All three filters are on the back of the tank. If you add pictus cats...or any catfish with large whiskers....this will deter the oscars from eating them. Raphael cats are also real spicky and the oscars dont eat them. But the pictus cats are much more active.