If it won't get in the way of the filter's impeller, fill one of your filter boxes with filter sponges at the bottom (coarse, then fine), and then a pack of bio rings at the top to build up some bio filtration- don't pack it so tight that it restricts water flow, though. Use the other one with disposable cartridges, if you want... or you could do it that way with both (keep some carbon cartridges on hand, in case you need to remove any chemicals from the water- spilled cleaners, medicines, etc). Rinse the sponges in tank water with your weekly water change- just stick them in the bucket, swish them around, and give them a few good squeezes. (put bio rings in the tank while you rinse out the filter boxes and sponges so they don't dry out- tank water only. No chlorinated tap water). Replace sponges as they become too dirty, or damaged. This will give you more biological filtration to handle the increased bio-load than the disposable cartridges alone, as you won't be throwing your colonies away every month, and there will be more material there to colonize. Get an air pump and a bubble wall, or large air stone, for the tank to add more air, make sure you don't go nuts with feeding, and you should be ok!