OWS Four Disc 9" Diffuser - Self Sinking BaseThe Four Disc 9" Rubber Membrane Diffuser offers a self sinking base, allowing it to sing to the bottom without having to add rock or sand to the base unit. The heavy gauge metal is textured with a powder coat finish. Rope holes are on the base to allow for easy insert and removal of the diffuser system. A back flow valve is present to prevent water from entering the airline.