Check valves have two ports and restrict the flow of fluids and other loose materials through a pipe or passageway in one direction. Flow enters through one port and exits through the other, closing when flow reverses or stops. Two common types of check valves are ball cone valves and swing check valves. A ball cone valve utilizes a spring-loaded ball seated against a cone-shaped interior surface to stop the flow of fluid in a forward direction, and a swing check valve has a movable disc on a hinge to block reverse flow.

Check valves are rated by their cracking pressure (the minimum upstream pressure required for the valve to operate), and they are used on various bead filters for koi ponds 
Valuable to protect your equipment. In aquaculture, air pumps are often below the water line. Use these Valterra check valves to keep water out of the pump with minimum head loss. Ask for specs. Made in the USA by Valterra.