Penn Plax Deluxe Bubble-Disk Airstone for Aquariums by Penn Plax

I use these in my koi pond to improve oxygenation and circulation to encourage the aerobic water cleansing bacteria. I have been able to develop a biological filter system, using these airstones to form a bubble driven recirculating vortex, that keeps the water beautifully clear and clean. On the negative side, the airstones tend to clog after a year or so, but they're the best I've found so far for such a reasonable price.

I use 8 of the largest size in my Koi pond and they are excellent. On the down side they tend to clog after about two years of use, but that is probably true of any airstone. They provide a very attractive stream of bubbles and very effective aeration to encourage the aerobic bacteria that purify and clarify the water. I am able to enjoy crystal clear water without any other means of filtration.