I would just add another HOB, maybe an emperor 400, and instead of running the carbon just fill some filter media bags with some matrix rocks and dump them in there. Id also put some sort of mechanical floss in there before the matrix rocks though so you wont have to clean the rocks a lot. Its easier to clean the mechanical media because you can just rinse that off in the sink and not worry about killing the BB, but the matrix rocks youd have to rinse with tank water or risk killing the BB with chlorine (which is a pain to do)
