Please DO NOT use them with CO2 system. The one way check valves are for common aquarium air pumps. Protect you air pump by preventing aquarium water from back siphoning during a power outage. Detachable design makes it easier for you to clean and repair. Unscrew to open it. Air Hole Diameter: 4mm/ 0.16 , Overall Size: 4.3 x 1.5cm/ 1.7 x 0.6 (L* D), Net weight: 4g. Package Content: 6Pcs Air Check Valve 6 Pack Black One Way Valves The non-return detachable check valves are made of durable plastic and designed to fit most common aquarium air pump while making no noise. Avoid getting water running into your pump when doing water changes or when the power is interrupted. Perfect for aquariums, fish tanks and gardening etc. Note: Don't place the air valves in CO2 system because the high pressure will make them crack and leak. Fits standard 3/16 inch airline hoses Detachable for easy cleaning An arrow to mark flow direction Good air flow & no restrictions Durable and aging resistant How to Install 1. Before use, clean and dry both sides of the tubing so as to hook up valve tightly. 2. Check the flow direction of the valve to make sure put on the correct sides. 3. Attach air pump check valve to the tubing carefully and connect completely. Tips: If the valve restricts air flow in both directions, please poke something into the hole on the inlet side to push open the valve. After that the valve would allow airflow in the right direction. Detachable Detachable design makes it easier for you to clean and repair. No tools needed and just unscrew to open it. Good at Prevention The air check valve protects you air pump by preventing aquarium water from back siphoning during a power outage. Features an arrow to identify direction quickly. Practical Application Fits for standard 3/16 inch airline hoses. Can be used for aquariums, fish tanks and gardening etc. But DO NOT place them in CO2 system.