The waterfall filters designed by Atlantic are up-flow filters. This means water flows from the pump into the filter through the bulkhead fitting and directly into the swirl chamber. Then the water goes through the grate and filter mat where solids are removed. For the final step, water flows through the media bag and filter material, and out over the spillway into your pond or stream. We recommend you fill the media bags with Savio SpringFlo, a great and inexpensive biological filter media.