There are three main types of filtration: mechanical, biological and chemical. Mechanical filtration removes particles from the water by forcing the water through a medium with holes. Particles larger than the holes get stuck in the filter and are removed from the tank. Biological filtration uses bacteria to transform ammonia into less toxic substances. Chemical filtration usually involves the use of activated carbon to remove chemical substances from the water. There are also certain specialized methods of changing the water’s chemical makeup such as reverse osmosis, protein skimming and peat filtration. However, these are special cases that are not used or required for the vast majority of freshwater tanks.
Many aquarium filters use all three types of filtration to clean the water, while others may use two or even one. A simple corner box filter performs all three functions and can be used as an example. The filter consists of a box containing a layer of activated carbon and a larger layer of filter wool. The filter wool is not actually wool, but is usually made of polyester batting.
Air enters through a tube that opens at the bottom of the box. The air bubbles rise through the layer of activated carbon and then through the filter wool before leaving through the holes in the top of the box. Because the air moves through the water, the movement of the air causes the water to move upwards through the filter.
The water carries with it chemicals and particulate matter. The activated carbon adsorbs some of the chemicals (Lenntech), which removes them from the water, while the filter wool traps some of the solid particles. On the filter wool live bacteria that convert ammonia into nitrite, and other bacteria that convert nitrite into nitrate. These bacteria are aerobic, which means that they require oxygen. The movement of the water allows oxygen to reach them easily and provides a constant stream of water containing the ammonia to be converted.
The box filter described here is one of the cheapest and simplest filters available. They work very well in small tanks, but other types of filters are more common in larger tanks.
Lenntech Water treatment & air purification Holding B.V. Retrieved March 3, 2009