This may sound like a dumb question but I don't know the difference between a cartridge and a filter. Because I read in the manual that I'm suppose to change the cartride every 4 weeks. But then I heard that most beneficial bacteria are in filters and we should not change it at all. So I'm a little confused as to what is the filter. The actual machine? The cartridge? The little sponge where the water gets sucked in to? Because the sponge I clean out.