Airline CO2 Tubing For Aquarium Hose Tube. 6mm (1/4") - Standard Airline Tubing Size .Dimension: 4*6mm (Inner diameter*Out diameter) This very flexible quality airline will fit all air pumps and fittings with standard 4mm outlets. Very light and soft, Professional oxygen pump hose for fish tank. Flexible silicone tubing that stays soft & flexible without kinking. High quality silicone tubing that does not crack or become brittle over time like regular tubing