Looked at it online
First you should put a pre-filter sponge on the intake if you haven't already. The cartridges are fine, don't change them until they disintegrate. You could put some ceramic rings behind the cartridge foams or you could just put some more coarse foam in there. Bacteria will colonize any surface and so it's just a matter of trying to maximize the surface area available for them to do so - ceramic rings are merely one of the most expensive and durable options, there are plenty of others including plastic army men which I've seen employed in a dump filter by one of CCA's more accomplished breeders. Certainly seems to be plenty of space in those filters to do something to boost its capacity.
