Aquarium Bottle Cap is designed for DIY live plant carbon dioxide diffuser air generator systems. It is easy to set up and features a simple, effective design. Equipped with a high-quality plastic O-ring inside the cap, it helps prevent carbon dioxide leakage. Made from high-quality ABS material, it is harmless, eco-friendly, and durable. The connector tube has an inner diameter of 4mm and an outer diameter of 6mm. This bottle cap is ideal as an accessory for DIY carbon dioxide systems and fits standard PET plastic bottles. Item Name: Aquarium Bottle Cap Material: ABS Size Details: - Connector Tube Inner Diameter: 4mm / 0.16" (Approx.) - Connector Tube Outer Diameter: 6mm / 0.24" (Approx.) Note: Due to differences in lighting and screen settings, the item's color may vary slightly from the images. Slight dimensional variations may occur due to manual measurement. Package Includes: 1 x Aquarium Bottle Cap