Outdoor Water Solutions ARS0026 7-Inch Airstone Diffuser This is a 7" diffuser airstone with a 1/2" male npt fitting that fits our airstone foot valve which is also called an inline backflow valve Note: shallow or larger ponds may require multiple airstone diffuser kits. Note: this air stone require an airstone foot valve to prevent water from backing up into the airline. ARS0026 This is a 7" diffuser airstone with a 1/2" male npt fitting that fits our airstone foot valve which is also called an inline backflow valve Note: shallow or larger ponds may require multiple airstone diffuser kits. Note: this air stone require an airstone foot valve to prevent water from backing up into the airline.