Sponge filters do this same task as bog filtration as well, at a much smaller scale. A sponge is shaped like a cylinder, then a 1cm hole is punched through the center. An air tube goes down the middle, and a 4cm "chimney" sticks up from the hole to improve flow characteristics. The whole contraption is immersed in water. The result is that the bubbles travel upwards, pulling some water with them, and exits the top of the chimney. This water is replaced by water outside the sponge. The sponge acts as both a mechanical filter, but also as a high surface area for microbes to live and process waste.<p>Bog filters work on a much larger scale, but sponge filters have been replacing "hang on back" filters in the aquarium hobby pretty rapidly over the last couple of years. It also helps that sponge filters only cost about $10 and have no moving parts