Features: brand new and high quality The check valves are for common aquarium air pumps. Please DO NOT use them with CO2 system. Great one way check for valve for aquarium and fish for tank. Protect you air pump by preventing aquarium water from back siphoning during a power outage. Made of durable clear plastic. Please use a paperclip to poke it from Input side to Output side when the outlet inside gets stuck. Suitable for 4 mm/0.15 inches i.d. oxygen tube. Easy installation, just insert the connector directly into the air tube. Specifications: Material: PVC Color: Blue/Black S/Black L/Transparent L/Red L Size: Suitable for 4mm/0.16in I.D. Oxygen Tube Weight (Approx.): Blue/Black S-3g Black L/Transparent L/Red L-5g Package Includes: 1 Piece Aquarium Check for Valve (Blue/Black S/Black L/Transparent /Red) note: 1.Please allow 0-1cm error due to manual measurement. pls make sure you do not mind before you bid. 2.Due to the difference between different monitors, the picture may not reflect the actual color of the item. Thank you!