I'm not sure what you should do but here's what I do:
I cut to fit a piece of prefilter material (what is this material called? Looks like the scrubber side of a sponge, is thick and it's white?) to fit into the slot of the cartridge. It was a little tricky as I had to cut out some of the black pieces of the plastic grid that were in the way and keeping the piece I had cut from fitting snugly into that area where the carbon was. I don't believe using carbon is at all necessary, except when after you have treated a tank with meds and want to remove the residual meds that may be in the water. Now when I do my weekly water changes I can easily pulll out that piece, rinse it in old tank water and put it back in. Your tank has the bio-wheel, correct? Most of your biological media is sitting on that wheel, rather than in your carbon filter "thingy". Using my replacement prefilter "thingy" also saves me money as I don't have to keep buying replacement filters, which fell apart after a month anyway.
(It's early here in California so my brain is having trouble finding my proper nouns.