KLOUD City Fish Tank Aquarium Non-return Air Pump Check Valve Package included 1 PCS Air Pump Check Valve It can prevent the aquarium from the water flowing back,which is caused by the decrease of CO2 pressure, and keep the function and service life of CO2 bottle. When used in the ozone generator, it can prevent the Ozone from flowing back to the air pump and eroding the propeller. It can protect the power head from the water flowing back which is caused by power cut.