4-Inch Filter Cup with Carbon / Media Chamber I originally designed this for myself, as I couldn’t find a filter cup that could properly handle loose media like carbon. My Aqua One ReefSys 320 has a small sump that’s already crowded with equipment, so this was a simple solution for adding carbon or other media where water needs to flow through it for proper effectiveness. After 32 versions and a lot of prototyping, I’ve finally achieved what I initially thought would be a simple design.