I googled Hagen Tropiquarium filters and couldn't get a picture of what the filters looked like. Would you be able to post a picture? I'll bet you can come up with something to use as a filter cartridge. If its made by Hagen I'll bet its a sponge/foam insert of some kind. Unless you really want to you can use carbon, but its really unnesessary. Its good to remove medications or help clear stained water from tannis leaching out of driftwood.
Do you mean "Fluval 2"?