Because Oscars are such sloppy eaters and produce a lot of waste the more filtration the better. I have 2 large filters plus undergravel filtration in my Oscar tank. Bio-wheels are nice but they suggest running them for quite awhile before introducing your fish. You may not have time for that if he needs out of the 33 gal asap. Be sure to use stress coat, stress zyme and put about a tablespoon of salt per 5 gals.