I love my sponge filters! The maintenance is next to nothing. You should not be spending that kind of money on a HOB filter. Carbon is not necessary to add to your filter media, and unless you are getting no water flow through the filter cartridge, you should not be replacing them. Simply swish your filter cartridge through some used tank water when it needs it, and until the cartridge is completely clogged where rinsing no longer helps, or it's literally falling apart, don't replace it! I've had the same filter media in some of my filters for 6 months!