Installed at the bottom of pond, this device encourages debris and waste to flow into a circular collection chamber, where it is carried away underground, via standard 3" PVC pipe (not included), to a disposal area. Use a pond bottom drain inlet to a skimmer: When the pond is 3-feet or deeper. When you want the ultimate in pond cleaning, such as when keeping koi. The bottom inlet creates a DUAL-FLOW filter box, skimming the top of the pond and pulling water from the BOTTOM of the pond...simultaneously. If you use underwater lights, the bottom inlet helps remove suspended debris that the lights would otherwise illuminate at night.