Official replacement biOrb Airstone. Each pack contains one airstone which should be replaced if the flow of bubbles within your biOrb starts to reduce. This normally occurs every four to eight weeks as the airstone will gradually block as part of the natural processes within the biOrb. The air stone, located at the base of the aquarium, breaks upthe flow of air into the aquarium into a mass of bubbles. After a time the air stone... biOrb Air Stone is a simple to replace stone, vital in the filtration process of your biOrb Aquarium. The airstone sits in the base of the biOrb underneath the filter cartridge. Its job is to help diffuse the movement of water and create the bubbles that flow up the bubble tube and disperse oxygen around the tank. Over time the tiny pores in the air stone can become blocked by dirt and lime scale preventing air from getting through properly. Replacing the air stone every few months will ensure your biOrb environment is kept to its best. Specifications 1 x Air stone.