clean during water change
You can clean that filter during the weekly water change. Draw some clean water out of the tank, maybe a gallon, then take the filter cartridge out and swish it really well, all the soft slimy stuff should come off, there will still be a brown tint to the fabric of the filter, and that is enough bacteria. Then go on and clean the filter itself, the input and output tubes, the impellor and impellor well. Dump all that water into your favorite plants. Then do a regular water change, clean the gravel if you need to (you probably do) and refill with correct temp water, correctly conditioned to remove chlorine or chloramine as needed.
