Check valves are essential when using a waterfall filter in conjunction with a pond skimmer. The check valve will prevent water in the filter from back-flowing into the skimmer when the pump is switched off and keep water in bogs when the pump is turned off and other purposes. Check valves are also essential when an external pump is being used. The check valve prevents the water from draining back into the pond or reservoir, keeping the external pump primed with water. Always make sure that check valves are installed in the correct direction.