Choosing your Pond Filter

Do I need a filter for my pond?
If you plan on stocking your pond with fish, plants, or a combination of both, then your answer is undoubtedly, "yes." Filtration is critical to maintaining the integrity of your pond and keeping it clean. A properly installed filtration system will pull water from the bottom of your pond and recirculate it throughout the pond. This movement of water prevents it from becoming stagnant and prevents pond turnover. Thermal stratification in ponds leads to pond turnover, which occurs when the uppermost layers of water suddenly drop in temperature and mix with the deeper levels of oxygen-depleted water. Such a turnover causes an overall loss of oxygen for fish and other aquatic life and often leads to pond kills. By cycling water through a filtration system, your pond's essential nutrients and oxygen will be distributed throughout the entire pond.

A filtration system for your pond should remove the toxins produced as a byproduct of the Nitrogen Cycle. Without filtration or well-aerated water, those toxins would lead to an unhealthy pond environment harmful or fatal to fish and plants. As food or organic material deteriorates or is excreted by fish, it forms ammonia (NH3) as a by-product. In the presence of oxygen and a filtration system ammonia will convert first into nitrites (NO2), and then beneficial bacteria will convert the harmful nitrites into useable nitrates (NO4). Plants and other aquatic life use nitrates to foster growth, and the cycle begins again. In the absence of a filtration system, ammonia will only be partially broken down into nitrites, which in excess will be harmful or fatal to your aquatic life. The diagram below illustrates the Nitrogen Cycle.

Even if your pond is solely ornamental, a filtration system will help prevent stagnation, inhibit algae growth, inhibit insect pests, and clean the water to be used in a decorative water display. Stagnant water tends to have a distinctive, unpleasant scent, which is generally undesirable for an ornamental pond.

What type of filter is best for my pond?
This question's answer is largely dependent upon the type of pond you have or are planning. There is one main factor that should influence your choice of filtration systems; whether or not you plan on stocking your pond with fish or other aquatic wildlife. We will discuss the functions of several types of filtration, as well as for what applications they are best suited.

Mechanical Filtration
If your pond is solely ornamental or you are looking for a pre-filter for your pump, then you will probably find the need for a mechanical filter. A mechanical filter is solely designed to separate large particles and debris from the water passing through it. This is particularly advantageous when protecting a pump. When run through a mechanical filter, water will not be treated and will retain any toxins or harmful substances it contained prior to entering the filter. As such, mechanical filters are not recommended as the only source of filtration for a pond containing fish or aquatic wildlife. Most filters available will feature at least some degree of mechanical filtration in addition to their specialized function. Skimmers, canister filters, and prefilters are common examples of filters that mainly rely on mechanical filtration.

A skimmer filter typically rests on the perimeter of your pond opposite the prevailing direction of current and wind to catch debris and large particles on the water surface. This prevents the debris and particles from falling into your pond where they would decompose and produce harmful nitrites and release them into the pond's water. Regular cleaning of skimmers is necessary to keep them at peak performance level. Skimmers are very convenient to maintain as they are at the pond's surface rather than being submerged. These filters only clean the water surface but may provide complete filtration for very small, shallow ponds.

Canister Filters and Prefilters
These filters are installed on the intake side of your pump to protect it from debris and large particles that could damage its impeller or clog your pipes. Canister filters usually consist of polyester or carbon coated foam pads contained in a flat housing that rests on your pond's bottom. A short length of hose leads from the filter housing to your pond pump. Another type of prefilter consists of a foam pad wrapped around a plastic tube, which fits directly onto the intake of your pump. Skimmers are often used as prefilters, catching larger debris than other prefilters.

Biological Filtration
If you are looking for a way to remove toxins from your pond water and to provide a safe and healthy environment for your aquatic wildlife, biological filtration is the best option for you. Biological filters house beneficial bacteria which help convert toxic nitrites into helpful nitrates, to be used by plants. Biological filters typically clean the water through mechanical filtration in addition to treating the water and removing toxins. Because of the dual filtration associated with biological filters, they may often be the sole source of filtration for your pond. Biological filtration may be achieved either naturally or artificially.

Natural Filtration
Although a created pond is never completely natural, you can utilize natural biological processes to filter and maintain a healthy balance in your pond. The most common form of a natural pond filter is a bog filter. Occupying 10% to 20% of your pond's surface area, a bog filter uses plants and natural materials to filter out debris and large particles while housing beneficial bacteria that remove toxins. Water leaches into the bog through a meshwork of pipes, where it is often met with a three to five inch layer of gravel to filter debris, and then passed through ten to twelve inches of soil and plant roots. The plant roots offer residence to beneficial bacteria and absorb the nitrates produced from toxic nitrites. Water leaves the bog through a shallow channel at the pond's surface after it has been cleaned and enriched. Like any filter, a bog filter requires periodic cleaning. The best way to ensure that you will be able to clean this filter is by installing a bottom drain. The bottom drain will allow you to flush the bog and remove debris. Here is a diagram which illustrates the basic principles of a bog filter.

Keep in mind that a natural bog filter requires an up front investment and routine maintenance which may not be well suited for the casual or novice pond owner.

Artificial Filtration
This type of biological filtration uses man-made media on which beneficial bacteria will embed themselves. This media usually has grooves, divots, or folds to provide maximum surface area for the beneficial bacteria. As water passes over the media, the bacteria convert the nitrite and ammonia from the water into nitrate, which is easily used by plants or other aquatic life. The media also catches large particles and debris, cleaning the water before it returns to the pond. Filters with artificial media tend to require routine maintenance and cleaning. These filters are readily available and are often the best option for the majority of pond owners.

Ultraviolet Clarifiers
Although not really considered a filter, ultraviolet clarifiers, or UV clarifiers, are most often used in combination with a filter or actually inside the filter itself. UV clarifiers are designed specifically to tackle the problem of green water due to water borne algae. By killing suspended algae, bacteria, and other microorganisms that flow through the clarifier, UV clarifiers return clear, safe water back to your pond.

The clarifier itself consists of an ultraviolet light bulb housed inside a quartz sleeve and contained within a filter or PVC housing. Any cells in the thin layer of water forced over the quartz sleeve will be bombarded with UV light and killed. As such, the beneficial bacteria growing on your filter media or on the pond bottom are never in danger of being harmed or killed by the clarifier because they do not pass over the UV bulb. Only suspended algae, bacteria, and parasites are affected. A common misconception is that string or filamentous algae will be killed after installing a UV clarifier. String algae typically cover substrate or lining, and so like beneficial bacteria, string algae are not affected as they do not pass through the clarifier. UV clarifiers do require seasonal maintenance. Be sure to check the quartz sleeve regularly to make certain the quartz sleeve is clear. Replace the UV bulb every season, even if it continues to light, because UV bulbs will lose their effective light spectrum over time.

What size filter do I need for my pond?
As a pond owner, you must first decide whether your pond will require complete biological filtration or, in the case of a small ornamental pond, simply require a mechanical filter. We will outline the requirements that will affect your choice of filter in each of these scenarios.

Complete Biological Filtration
Whether using natural filtration or artificial biological filtration, your pond will need a filter capable of cycling its entire volume once per hour for a maximum effect or once per two hours at a minimum. In the case of natural bog filtration, this would mean creating a filter representing 10-20% of your pond's total surface area and using a pump capable of cycling the pond's volume through the bog once per hour, or once per two hours at a minimum. When choosing an artificial biological filter, simply ensure that your pond's volume falls within the manufacturer's recommended range and that you have a compatible pond pump. UV clarifiers should also be chosen in accordance with the manufacturer's recommended pond size.

Mechanical Filtration
In the case of most mechanical filters, their size is proportional to the amount of maintenance they will require. Foam prefilters and canister filters, for example, are mainly designed to protect a pump's impeller from damaging debris and large particles. A small prefilter will need to be cleaned more often than a larger one as more debris may accumulate on a larger prefilter without reducing or blocking the flow of water to the pump. Skimmers, however, are sized proportionally to your pond's surface area. The manufacturer will recommend the pond surface areas best suited for each skimmer.

How do I calculate my pond's volume?
Calculating your pond's water volume can be intimidating to many people, but by using a simple calculation and conversion, most ponds' volume can be easily determined. Every cubic foot of space in a pond is occupied by 7.5 gallons of water. Therefore, your pond's water volume may be determined by multiplying its total volume in cubic feet by 7.5 gal/ft3. Not every pond has a shape whose volume is easily calculated. Below we offer calculations for determining the volume of several different pond shapes.