Simple Aquarium Bottle Cap for DIY Live Plants CO2 Diffuser Air Generator System. Description: Aquarium bottle cap is designed for DIY live plants carbon dioxide diffuser air generator system. It is easy to set up, and features simply and excellent design. Attached with high quality plastic "O" ring inside of the cap, the bottle cap can prevent missing the carbon dioxide. Made of high quality ABS material, it is harmless, eco-friendly and durable. The connect tube head inner diameter is 4mm, outer diameter is 6mm. This bottle cap is very suitable to be DIY carbon dioxide system's accessory that the size fits common PET plastic bottle. Item Name: Aquarium Bottle Cap Material:ABS Size Details: The Connect Tube Head Inner Diameter: 4mm/0.16" (Approx.) The Connect Tube Head Outer Diameter: 6mm/0.24" (Approx.) Notes: Due to the light and screen setting difference, the item's color may be slightly different from the pictures. Please allow slight dimension difference due to different manual measurement. Package Includes: 1 X Aquarium Bottle Cap