Behind every clear water pond is an effective filtration system. Filters collect debris and fish waste, keeping it out of the pond and away from the algae and filters also give nitrifying bacteria more surface area on which to grow which also helps to control algae. They can cost anywhere from $75 for a small
do-it-yourself filter to many thousands of dollars for large ponds. If you are having problems with green water or string algae you probably have an under sized filter or no filter at all. . Filters for small ponds can be in ponds, but ponds over 500 gallons require more surface area than you want taken up in a pond and so they should be an out of pond filter. There are two parts to filtration. Mechanical filtration and biological filtration. Each serves as an important part of keeping your pond clear and healthy.

  • Mechanical filtration collects debris (such as leaves, sediment, food etc.) keeping it out of the pond thus improving the clarity of the water and the health of the fish. The mechanical filter media can be made of many different things: but is commonly made of scrubbing pads, Matala or other porous material. Many filters serve as both a mechanical filter and a biological filter. In small prefabricated ponds a fibrous sock is placed around the pump to collect solids. With larger capacity pumps in larger ponds that is not practical to do.

  • Surface skimmers go a long way to keeping the pond clear and should be used in conjunction with mechanical and biological filters. They draw leaves, food and other items on the surface into a self-contained mechanical filter, preventing the organics from sinking to the bottom and decomposing which in turn feeds algae. The skimmer can be part of an out of pond pump system or have it's own pump and separate discharge. Skimmers should be placed at the opposite end of a stream or waterfall so organics are pushed into the draw of the skimmer. Skimmers are not very effective if water lilies or other floating plants cover the surface of the pond as they impede the flow of surface water.

A do-it-yourself filter - 30 gallon tote using
​scrubbing pads, cleans a 500 gallon pond.

Amount of debris a mechanical filter 
​picks up in 2 weeks in a clean pond.

Drum filters clean very fine particles from the water and are self cleaning

  • Biological filtration gives a pond much more surface area for nitrifying bacteria to grow. It is this bacteria that breaks down the ammonia and nitrites in the pond, making it safer for the fish and helping to keep the water clearer. The media can be made of many types of material, including scrubbing pads, poly strapping, Matala and bead filters. Mechanical filters often do double duty and aid in biological filtration, but can require frequent cleaning.

  • ​Bog filtration uses plants to absorb add surface area for nitrifying bacteria and to absorb nitrates which are the end product of the nitrogen cycle. Putting a bog between the pump output and the pond helps to keep the green water down often seen in ponds. It is important to spread the water out which slows it down and allows the plants to more easily absorb the nutrients.

  • UV filtration passes water around a bright fluorescent  UV tube housed in a pipe to kill floating bacteria that make water look green. This filter will not kill string algae and will not harm fish or plants in the pond. While the light may glow, the intensity drops after two years and the tube should be replaced. If you have kept the organics out of your pond and your pond is still green this filter may be a good option in helping keep your pond clear.