You raise some good points. Even a screen that covered the entire end of the tank may not have enough surface area to prevent clogging. And I agree that having the extra bacteria in a foam filter doesn't necessarily hurt the tank. I was just hypothesizing a way to eliminate the main objection to a mattenfilter, the room it takes up in the tank. But I'm not sure the idea is worth the expense of experimenting. Screens fine enough to do the job aren't exactly cheap.