Undergravel Filters
Undergravel filters consist of plastic filter plates which are like grates that lie under the gravel of your fish tank. These plates allow water to flow freely under the gravel. Water is drawn through the gravel (which acts as a mechanical and biological filter) by the use of either a power head which will pump water out of the top of the lift tube and into the tank or an air pump which will blow bubbles at the bottom of the lift tubes and the bubbles will lift water up the lift tube and into the tank. As water is moved out of the lift tube, it is replaced with water from under the filter plate, which in turn pulls water through the gravel where it is cleaned. The gravel it self provides mechanical filtration by catching large free-floating particles. The gravel, as well as the filter plate, tank bottom, and lift tubes, provide a bed for the bacteria of a biological filter. Under gravel filters primarily provide biological filtration.
