I'd like to know what kind of filter you're using first, but here goes anyway, lol. IF it's anything other than a cannister filter, I'd say go with more foam. The problem with filter floss ( I remember when it was glass wool, fiberglass) is that when you replace it, you're throwing away all of the beneficial bacteria that have colonized on it. With a sponge replacement for your carbon, you can simply rinse the foam out in syphoned off water change water, put it right back in, and not lose any of the beneficial bacterialogical action. If you're using a cannister filter, that already has many layers of biological action already, then you can use the floss as a water "polishing" agent.