Well, the experiment is over for now. Over the course of a couple days I put in a 3# bag, plus the first 9 oz. bag, and still no change in pressure. I have to conclude that the filter is way, way, way too big, and I'm not going to shell out so much money just for that perceived 'sparkle' and 'polish'. I've been using the pool for 5 years now without it, and I've somehow managed to enjoy it anyway. Thanks for your input chemgeek.