Nitrites are then converted to nitrates, which are converted by the plants to food, the fish eat the plants, and the biological cycle is completed. All surfaces in the pond covered by water function as small biological filters, because the bacteria that accomplish the modification of ammonia to nitrite and then to nitrate cover every available surface. A biological filter, added by the pond owner, is simply a place that is very hospitable for these bacteria to live. Also, because the bacteria cover the surfaces of objects in a one-cell layer thick sheet, the filter should contain large amounts of space for the bacteria to coat the media in this type of filter. It is important to understand that it takes from one to three months for the bacteria to colonize the available surfaces, that the bacteria cling to the surfaces tenuously and are easily dislodged, and that they must always have water over them and oxygen present.
Surface area is the most important thing.
There are numerous types of bio-filters available on the market. Lava rock is a commonly used material, and plastic beads are becoming popular as well. All filters require the pond water be pumped through them. For efficient filtration a certain volume of water, based on the size of your pond, should be pumped hourly. To determine this ratio for yourself you must first figure out how much water your pond holds.