A check valve assembly is needed to connect your pond pump to the pond plumbing. Two most common check valves are the Dual Union Check Valve and the Swing Check Valve.
Dual Union (meaning locking collar on both ends) Check Valve has a removable connection underneath the check valve which allows you to disconnect the pump and plumbing without draining the water in your filter.
Swing Check Valve is a specialized PVC fitting used to allow water flow in one direction through a pipe. These PVC check valves are the "flapper" type without springs. Swing check valves do not get clogged with algae. An example of use is to locate this valve between the discharge side of the pump and the biological filter. If power is lost at the pump, a check valve will automatically close preventing unfiltered water from backflushing into the pond.