I use those pond filter boxes they sell at home depot. They have 2 densities of sponges in them. They're made to actually have a pump inside them, but since I'm using an external pump, I just stick the intake hose into the box. I'm going to rig up a Y on the intake so I can use two of them in the spring, and I'll have a third clean one so I can rotate them once a week, so each one gets cleaned every other week,