Wednesday, May 18, 2011

Undergravel Filter

The undergravel filter was once the standard for biological filtration. This type of filter consists of perforated plastic plates that sit on risers about an inch above the bottom of the aquarium. Two to six plastic uplift tubes fit into holes in the base plates. In the simplest design, an airstone at the bottom of each uplift is supplied from an electric air pump. As bubbles rise in the uplift, water is pushed along. This causes aquarium water to flow downward through the plastic plates. Newer models use a small water pump, known as a powerhead, to suck water up through the lift tubes, providing better turnover. Powerheads can be adjusted to regulate the speed of water currents produced. The outlet often swivels to direct the water flow as desired.

With this arrangement, the substrate itself is the filter medium. Oxygenated water flowing through the gravel brings ammonia to the beneficial bacteria living on every grain. Mechanical filtration results when debris becomes trapped in the gravel bed.

A reverse-flow undergravel filter draws water through a mechanical filter, then passes it back down the uplift tubes to be distributed throughout the aquarium after it is pushed up through the gravel bed. A reverse-flow system increases the efficiency of the undergravel filter and the longevity of the bacterial colony, primarily because it reduces debris accumulation that restricts the water flow.

The major drawback to both undergravel designs is that debris accumulates in the gravel bed. Eventually, regular vacuuming (using a special aquarium vacuum tube) of the gravel becomes necessary to maintain an optimal flow, regardless of the direction the water moves.

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