People who have kept fish for sometime would be familiar with the under gravel filter filter system. Before modern filtration methods became adopted this was one of the number one choices. Though it is not a new technology the undergravel filter is still a viable filtration method on it's own or coupled with a second filter type.

Under gravel aquarium filters use the substrate as the filter media. Water pases over the gravel which is coated in useful bacteria. These microbes process the fish waste in the water. Water is drawn into the undergravel filter due to the uplifts. These are powered by air pumps or sometimes power heads.

A thick layer of gravel is best used with an under gravel filter. Use a medium gravel size are pile it 2 inches high over the filter plates. From time to time you may need to remove particles trapped in the surface gravel. When doing so use a gravel vacuum and try to disturb the gravel as little as possible. This should be the only regular maintenance required for this type of filter. Every 6 months or so you will need to strip back the entire aquarium and thoroughly clean under the filter plates.

A variation on the traditional undergravel filter reverses the water flow. By doing to the gravel does not trap solids but biological filtration is still active.