I always used to use the Marineland Bio-Wheel filters, but just got two AquaClear ones. For bang for the buck, I don't think you can beat a Penguin 350 ($27 w/ pricematch at petsmart), but I think the AC's are better filters. I don't use cartridges in my penguins and emporer, but instead I buy AC110 foam and cut it up to fit in the chambers. A lot better and less expensive than changing cartridges, IMO. You don't have chemical filtration that way, but I don't see a need for it in most cases, anyway.