As a general rule, a pond needs a pump that circulates ½ of the total pond volume every hour. This rule applies to ponds with a fish stock rate of 1” of goldfish or ½” of koi per square foot. Any more fish will require more circulation and filtration. Water gardens that are strictly plants with only a few fish may need no pump or filter at all. Filtration is also highly dependent on the fish that is stocked in your pond. Koi ponds and high stock rates require larger and more sophisticated filter systems. Some ponds with light to moderate stock rates can simply be filtered with a sponge-type filter pad, which may need regular cleaning to prevent clogging of the pump.
Many newly installed ponds lack surface area in which beneficial bacteria can colonize, especially
pre-formed liners. Biological filtration is a technique that involves the use of surface area for beneficial bacteria colonization
to rid the pond of excess nutrients. Biological filters usually consist of large canisters filled with lava rock, sponge, or
any other media that supports bacterial growth. Some models even feature a planting area on top for water plants to further
assist in filtering. Adding a layer of sand or gravel to the bottom of the pond can also help in biological filtration.