Aqua One Check Valve
Prevents water from flowing back into your air pump motor. This can often be the case if a power outage occurs.
The "in" side of the check valve should be nearest the air pump, and the "out" side nearest the fish tank. Cut a short length of air tubing to go from the pump to the check valve, and have more air tubing go from the check valve to the air stone, or whatever air device is in your tank.