Mechanical filtration is the primary type of filtration for swimming pools, but should be used only as supplementary filtration on a pond. Mechanical filtration physically stops debris so it can be removed. It does nothing to remove liquid or semi-liquid wastes. In a pond it is primarily used in skimmers or prefilters to keep debris out of the pumps. On its own it is insufficient to keep a pond healthy.
Chemical filtration is commonly used in aquariums but typically used in ponds only for special circumstances. Chemical filtration consists of running the pond water through a bed of media which removes undesirable chemicals and/or toxins by bonding them with the media. The most common types of chemical filtration are activated charcoal to remove chemicals, medicines or tannic acid discoloration of the water and zeolite (ammorocks) to remove ammonia. Chemical media needs frequent replacement or regeneration, as it “fills up”.
Ultraviolet filtration is highly recommended for both planktonic (microscopic floating) algae and disease prevention. As the water passes in front of a UV light, the light kills whatever is in the water. There are several key things to keep in mind with a UV installation. The first is that you should have at least 1 watt of light per 100 gallons. Next is flow rate. If the water passes the light too fast, the light does not have enough time to do its job. Some lights are marketed as clarifiers, some as sterilizers. A clarifier will kill algae but not disease organisms, where a sterilizer will do both. The only difference in reality is flow rate. Slow down the flow through a clarifier and it becomes a sterilizer, speed up the flow through a sterilizer and it becomes a clarifier.
Vegetative filtration is simply running the water through plants, either floating or planted. All plants will help pull the nitrates and phosphates from pond water. Nitrates and phosphates are fertilizers and can promote excess algae growth. The most efficient are floating plants, then bare root plants, then plants in mesh pots and least efficient are plants in hard pots. Bare root plants are most efficient when water is forced through the gravel they are in.
Biological filtration is the most important type of pond filtration. Water is run through a bed of media that nitrifying bacteria grow on. The bacteria break down and digest the organic wastes in the water, both liquid and semi-solid. The filter media can be mats, bioballs, lava rock or anything similar. The bacteria grow on all the available surfaces. With constant water flow, aerobic bacteria grow. Without constant water flow through the media, anaerobic bacteria replace the aerobic. Anaerobic are only about 10% as efficient as aerobic bacteria and generate hydrogen sulfites, which stink. A biological filter should ideally be cleaned no more than once a year, as cleaning will kill your bacteria and it takes three to six weeks to fully re-establish the bacteria colony. The bacteria colony, or culture, also has a tendency to weaken over time. Adding fresh bacteria with enzymes will constantly refortify the microbe culture.
Types of Biological Filtration
Submerged filters– these typically come with small pond kits. We do not recommend them as they require a great deal of maintenance and aren’t particularly efficient. Typically need to be cleaned up to several times a week.
Pressurized filter– similar to a bead filter, but typically will have filter pads inside. Comes apart to clean the inside. Good for smaller ponds where a waterfall or bog is not practical. Usually need monthly maintenance.
Bead filters– very efficient but also expensive. These look very much like swimming pool filters, with a bead type of media instead of sand. They are best for formal ponds or strict koi ponds (as opposed to water gardens). As they do not remove nitrates, partial water changes are recommended for ponds with beads filters as their primary filtration. Hard to hide unless a pump room or well is built and needs to be backwashed at least monthly. Also, not very efficient at controlling particulates.
Waterfall filters– our preferred type of filter. If large enough (that is, if it holds enough media) it can provide all of your filtration. With rocks and plants it can be hidden and so provides a very natural look. Should need to be cleaned only once a year.
Bog filter– the best. If you have room, it combines the best of biological and vegetative filtration. A bog equal to 20% of the surface of the pond will provide sufficient filtration for even a crowded pond. A bog filter should be shallow – it does not need to be more than 12” deep. It should have a grid network in the bottom so that water comes up evenly throughout the gravel in the bog, so as to prevent anaerobic bacterial growth.
Gravity filters– sort of a cross between a waterfall filter and a pressurized filter. Potentially very efficient, but hard to hide. Excellent for quarantine or holding tanks or ponds where a waterfall is not practical. Much less maintenance than a pressurized filter.
Aeration is not true filtration but an adjunct to your filtration. In a typical pond with a skimmer and waterfall, only the top 12” to 18” of water gets moved and filtered. The water in the bottom of the pond tends to stagnate and get de-oxygenated. Adding airstones to the pond bottom causes water from the bottom to constantly move up into the filter stream where it can get oxygen and be filtered, also bringing small particulate matter up with the bubbles to get picked up and removed. Additionally, aeration may become critical in hot weather. The warmer water is the less oxygen it holds, while the fish’s metabolism speeds up and more oxygen is needed. Airstones help insure that the water is holding the maximum possible amount of oxygen. Anytime fish are seen at the surface gulping for air you should start worrying. Oxygen deprivation typically kills the biggest koi first.
Aeration also helps in the winter in keeping part of the surface ice free for gas exchange. Some books and on-line sources will tell you not to mix surface air/water with the deeper water during the winter, but that simply is not a concern in our area.