2pcs STAINLESS STEEL CHECK VALVE for air co2 line 4/6mm tubing Regulator Diffuser plants tank STAINLESS STEEL CHECK VALVE (4cm) x2 ONLY for Co2 Tank. !! ONLY for Co2 Tank !! The valve is STAINLESS STEEL anti-high pressured, anti-acid and alkali, prevents from water flow return. Suitable for CO2 supplying system. for 4/6mm tube