Penn Plax AS6Q 4-Pack Aqua Mist Air Stone Cylinder Aerator for Fish Tank PACK OF 4 AIR STONES: sold as a 4-pack of fine mist aerators. STEADY FLOW OF BUBBLES: which will provide a better supply of oxygen to your fish, and a beautiful look to your aquarium. SMALL SIZED MIST AERATOR: measures at a 7/16 inch diameter (or 1 centimeters). SHOP PENN PLAX FOR ALL YOUR AQUARIUM NEEDS: Penn Plax sells a wide variety of Aquarium products; such as tanks, filters, decorations, and much more