What is an under-gravel filter for Aquarium?

An under gravel filter uses the gravel bed of the aquarium itself as the filtering material. One type of under-gravel filter is comprised of a sheet of perforated plastic which is placed in the empty aquarium, the gravel being put on top of it. The air-lift tube is located in one corner. It draws water from under the gravel to above the water surface, thus causing water to circulate through the gravel.

To date no one has given a suitable explanation for the satisfactory operation of this type of filter. The sludge and dirt seem to melt away and disappear. The effectiveness of this filter is no doubt involved with the action of bacteria, those minute organisms which thrive on decomposing matter. Some bacteria can only live without air; they are called anaerobes. One by-product of their action is hydrogen sulphide (H2S), a poisonous, foul-smelling gas, the one giving rotten eggs their characteristic odor.

Gravel that has decomposing matter in it will soon harbor colonies of anaerobic bacteria. It is probable that the circulation of water through the gravel—induced by the under-gravel filter—destroys the anaerobes and permits the establishment of aerobes which aid in the decomposition of the organic matter without the toxic by-products associated with anaerobes. While this has yet to be proven, it does provide a logical explanation for their effectiveness.

Unfortunately the length of time for which this type of filter is effective is limited. In practice it has been found that the small holes through which the circulation is carried on soon plug up with dirt. For continued usefulness, a bottom filter should be uprooted every six months and cleaned thoroughly.