Rhinox Plastic Check Valve - Aquarium CO2 Diffuser Regulator Air Pump Tubing You are winning 1 pcs of Check valve Place in your airline tubing between your air pump and aquarium water to prevent water returning down the air line Every air pump & Co2 solenoid should include with one of these for each outlet but most don't come with them An essential accessory for every air pump. Provides protection during a power or pump failure. Installation: Simply cut airline at convenient location between air pump and first outlet.