I didn't think you had any choices other than those little cartridge thingies--which already have carbon in them--(which is why very few saltwater hobbyists use them). If they do have some type of extra resevior then I would use something biological like stars or ceramic rings, or the plastic balls, whatever will fit in there.
Is this your only source of filtration on this big tank???????? If it is be very careful about stocking--you'll need to stay VERY conservative.
Good Luck