Many pond builders have found that adding a bottom drain helps keep a pond cleaner. Fish waste and other muck that accumulates on a pond bottom can be easily removed by a bottom drain. Ponds that are over 2' deep can become stagnant without a bottom drain.
Installed under the liner, flex PVC pipe carries the water under the liner to the skimmer box. Bulkhead fitting is installed in the skimmer, a PVC male adapter connects the flex PVC to the bulkhead fitting. The valve is placed inside the skimmer box to control flow. When the pump is turned on, water will flow into the skimmer as well as the bottom drain.