HIGH QUALITY BRASS VALVE - Rhinox Brass Check Valve is made from sturdy brass that doesn\'t bend, break or rust, unlike plastic check valves. ANTI-LEAK, ANTI-BACKFLOW - When installed correctly (check the arrow on the piece to know the correct side), Rhinox Brass Check Valve ensures that the water and air circulations are consistent and going in the right direction without any backflow. Great for sensitive fish species. FOR CO2 SYSTEMS ONLY - Made specially for CO2 systems and NOT air pumps. Fits tubing with 4mm inner diameter and 6mm outer diameter for best results.