These High Quality air stones are made of harmless mineral material.With high temperature sintering, the stones are sturdy and durable, have high dissolved oxygen rate. Disc Shape Air Stones can stay steady at the bottom of your tank. They Gradually diffuse air into the tank and provide oxygen for fish to breathe and plants to thrive and adds to the beauty of the aquarium. Remember that any air stone will need to be replaced every few months as they get clogged with dust, bacteria, and algae. Soaking the air stone in water for one hour before use.Brush the stone surface every 2 weeks, you will find that it can last for a long time and save a lot of money. Package Includes: 6 x Air Stones Size: 100 mm/4". FAST FREE AND DISCREET SHIPPING !! BUY MORE AND SAVE BIG $$ ON SHIPPING ! !Suitable for inner diameter 4mm soft tube.