To water backflow to damage the front end of the pump or other products and equipment Package Content: one-way check valves Note: Please be sure to connect the end with " OUT " to the end near the air stone Each outlet needs to connect a stop for, two-headed pump needs two stop valves, and so on After the addition of the stop for, the pump can be placed or below the surface of the water to the risk of water backflow when the pump is shut off, Manufacturer: Uqezagpa