I have not changed the filter on my 10 gallon for over 2 years. Its just not necisarry and everytime you remove one of the two, you loose half your bacteria, and have a mini cycle(this may be your problem?). If you gently swoosh the filter arround in a bucket of old tank water during the water change, you will clean off the "crud" without loosing much bacteria. Now as far as removing the filter per a treatments recomendation, good that you read the directions, but next time try a salt/heat treatment if you get ICH. You wont need to remove the filters, and the salt will come out eventually with water changes. The only reason you would want to use carbon(as stated above) is to remove meds from the water. The carbon will basically loose its function over time, but will still serve as a bed for your bacteria, so you don't need to go but new filter media, just leave the ones you have now.