not saying either is wrong, and your sponge filter will offer some living space for bacteria, but you can add so much more filtration with a type of bio-media in the tank, carbon is not bad media at all but is more often needed for removing certian chemicals from the water and it does remove some waste but a good biological filter will do more good to your fish. as said before you can still run carbon if you like and many do, but there is no reason at all to have a whole canister full.