Use the "default" sponges around the outside, and add as much bio media (Matrix, rings, etc) as possible, as long as the filter can be reassembled properly. Be sure to leave a "little" space so that the baskets "stack" properly, lube your orings with silicone grease(Lowes/Home depot) and DON'T over-tighten the fasteners.
Even as little BIO media as it seems the FX6's hold, trust me you don't need much more than that, and if for some reason you did, you add another FX6, or even a cheaper alternative second canister filter(Sunsun 304B) or HOB(Aquaclear 110)or the like.
Also, a prefilter sponge to replace the intake "screen Unit" helps to majorly prolong maintenance intervals, however you will have to pull the prefilter off and rinse it once per week(mine can last as long as 2-3 weeks but I only feed my fish once every other day). Less food is better, think about how they feed in NATURE, they don't have "fat" bellies in nature every day, and it's been said it's generally healthier for them).
If it requires it more than that, you are LIKELY overfeeding or overstocked....