Standard Air Tube ID: 0.4cm.Function: Connect it to the air pump and put it into the water to help the air pump pump oxygen into the water.Features: The stone High temperature sintered air is made by high temperature sintering, which is different from general commercial air stone. The material is sturdy and durable, it can be washed repeatedly and used for a long time. Medium bubbles: The air stone disc produces dense and abundant bubbles. A great way to increase oxygen solubility in your aquarium and enliven aquatic life. Reliable Material: Made of mineral material and sintered at high temperature. Ideal for aeration in a DWC tank. Helps maintain healthy fish or root growth. Water Stable: Each disc measures 4D x 1.5H (cm). Its round, flat shape allows it to always sink to the bottom and sit securely in the tank. Easy to use: Clean it regularly with a brush as it gets clogged after weeks or months of use. Instructions: 1. Please use an air tube to connect the air stone. 2. Please soak this product in water for 15 minutes before use. 3. For long-term use, if dirt is blocked and air bubbles are reduced, it can be removed for cleaning and repeated use. Set of 4.