This is where the bulk of the bioconversion occurs. This is a basin dug out of the top of the mound behind the pond, and lined with the same rubber liner that lines the pond and waterfall. I made a frame out of 0.5" PVC as a rigid support for the liner, which I sewed to the frame. The outlets from the pump go to the bottom of this pool, embedded in some "lava rocks": Balance kiln fired bio-media 25 lb bag $29.99 from Koi Joy. On the filter pool I float some water hyacinth; a half-dozen little ones quickly multiply and become a giant mass, fed by flowing water and protected from goldfish nibbling. I tied a piece of plastic window screen to the front of the frame to hold back the plants. It works: after some fluctuations in water quality as Winter turns to Spring, I get no detectable ammonia or nitrites all Summer.