I, too, would get some coarse sponge material like AC 500 replacement sponges and stuff your filters with it. I have aquaclears and just rince my sponges every couple weeks (I have them prefiltered) by rinsing them out in a bucket of tank water. works great. No problem with using the cartridges though. Just rinse them out until they get really nasty and fall apart then replace one at a time. Your bacteria should be totally fine if you only replace one per week or less.
