Perhaps the most versatile and efficient of these alternative approaches to biological filtration is the sponge filter. As the name implies, such units employ a synthetic sponge as a substrate for nitrifying bacteria. The internal structure of this material provides these bacteria with an enormous area to colonize. In its simplest version, such a filter uses a stream of air to create a current that pulls waste-laden water through a sponge mass. Here, metabolic wastes and suspended organic matter are broken down by nitrifying bacteria.
More sophisticated units employ a small water pump to accomplish the same objective.
A wide selection of air-driven sponge filters can be found on the market. It is thus possible to purchase or create sponge filters adequate to the needs of tanks with capacities of 5 to 125 gallons. When selecting a sponge filter, bear in mind that two factors determine operating efficiency. The first is the dimensions of the sponge mass. The greater the surface area exposed to water, the more effectively it will function.