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Improving Water Quality with a Biological Pond Filter

A quality biological pond filter is an essential piece of equipment necessary to any well-designed filtration system. Biological filtration is a broad term referring to the means of housing beneficial bacteria that are able to clean pond water at a molecular level. In a natural stream or river, bacteria colonize on the stones and pebbles at the bottom of the stream, which naturally break down toxins as the water flows overtop. It is this natural process that manufactured biological filters attempt to emulate. However, instead of cultivating bacteria at the bottom of the pond like in nature, for ease of maintenance, bacteria are colonized in various types of filter media, which are stored in a filter housing outside of the pond. The water is forced to flow through the filter via a pumping system, or a gravity fed system.

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Why You Need Bioloigcal Filtration

One of the main uses for a biological pond filter is to convert harmful ammonia, which is toxic to fish and other life, into nitrite and then eventually nitrate. Amonia is naturally produced from fish waste and other decaying matter in a pond. Although nitrate is non-toxic to koi and other pond fish, it is a plant nutrient, which will naturally promote the growth of certain types of algae such as blanketweed. Therefore, it's a good idea to try to manage the levels of nitrate in your pond. There are a couple ways of dealing with excess nitrate. One way is through the use of a vegetable filter, which is a separate area of the pond filled with moisture-loving plants that will naturally absorb nitrate. Another way is through scheduled partial water changes, which replaces a portion of the pond water with new nitrate-free water.

Types of Biological Pond Filters

When choosing a biological pond filter these days, a pond keeper is faced with numerous styles and brands. Here are a few of the most popular categories:

Gravity Return Filters

Gravity Return Filter

Gravity return filters have been around for sometime and are one of the most cost effective biological filtration systems available. Water from the pond must be pumped into the inlet of the gravity return biological pond filter using either a submersible or external pump. The water then flows through the filter, eventually returning to the pond via gravity. Because of this, the filter must be elevated above the surface of the pond for gravity to pull water from the filter back into the pond. This feature happens to also be one of the complaints associated with gravity return filters; they must be elevated above the pond level, and are therefore harder to conceal.

A pressurized pond filter

Pressurized filters

In an attempt to solve the above problem of the biological pond filter having to be elevated above the surface of the pond, pressurized filters were developed. With a pressurized filter, the water is pumped into the inlet much the same as a gravity return filter. However, unlike a gravity return filter, the water leaving a pressurized filter remains under pressure on exit. This means that the filter does not have to be elevated above the pond, and can be buried to better hide the filter box.

Gravity Fed Multichamber Filters

Until recent years with the development of fluidized bed and bubble bead filters (discussed below), gravity fed multichamber filters were the de facto standard in medium to large size koi ponds. Gravity fed filters, like the name suggests are fed by gravity via a bottom drain at the bottom of the pond. The top of the filter is placed exactly level with the top of the pond, such that gravity will naturally equalize the level of the water in the filter to be the same height as the rest of the pond. The last chamber of the filter contains a submersible pump which pumps filtered water back into the pond. Multichamber filters work well due to the multiple stages that the water must pass through before finally being returned to the pond. Each stage contains a unique filtration method, methodically refining the water quality further as each stage passes. The first stage of such systems is often a Vortex filter, which filters out heavy solids including fish waste, leaves, and hair algae.

The main complaint of gravity fed filtration systems is similar to gravity return filters – they take up quite a bit of space and must be installed at certain elevations to work properly. Often times gravity fed systems are installed beneath a wooden deck of some kind next to the pond, effectively hiding the filtration system from view.

Fluidized Bed Filter
Fluidized Bed Filter - Water enters
into the bottom of the chamber
under high pressure causing the
coral sand to swirl or 'fluidize'.
The high amount of movement
provides a great place for both
biolgoical and mechanical filtration.


Fluidized Bed Filter

Fluidized bed filters are one of the newer advancements in filtration technology. The main difference is that with regular biological pond filters, the filter media is stationary inside the filter. In the case of fluidized bed filters, a special type of granular media is used which occupies the bottom portion of a flooded chamber. Water is pumped into the bottom of the chamber, causing the filter media to fluidize and expand while swirling inside of the chamber. This constant motion of the filter media is what distinguishes fluidized bed filters from traditional biological filters. The biological filtration process takes place as the bacteria housed within the granular media interacts with the flowing water. The water eventually makes its way through the abrasive filter environment, to the top of the chamber where the water is returned by gravity to the pond.

The main advantage of fluidized bed filters is that the granular filter media contains a much larger surface area to house bacteria than any traditional filter media. This allows a fluidized bed filter to be much smaller than a traditional filter, while achieving the same, or a higher level of filtration.

Bubble Bead Filter

Bubble Bead Filter

A bubble bead filter is a type of biological pond filter that uses small plastic beads (3x5 mm in size) as a filter media. The plastic beads float in the top part of a chamber, which houses bacteria for biological filtration as well as mechanical filtration. Mechanically, the beads are able to filter out particles as small as 15 to 20 microns. For comparison purposes, a grain of sand is around 50 microns.

There are a couple main advantages to bubble bead filters. The first is that most models incorporate a backwash-cleaning feature, which makes cleaning the filter much easier than traditional filters. Another common feature is the addition of an integrated UV filter to control green pond water caused by algae growth.

Filters Combining Multiple Methods of Filtration

Certain manufacturers have created hybrid filters that incorporate 2 or more forms of filtration. One such example, and also one of our favorite filters to recomend at is the Aqua Ultraviolet Ultima II filter which incorporates biological and mechanical filtration into one unit. In recent years, newer filter technology such as the Ultima II, as well as fluidized bed filters and bead filters have become increasinging popular in koi or other medium to large scale ponds.

Other Kinds of Filtration

There are many other kinds of non-biological filtration that are useful in a pond. Such examples may include UV Filters, skimmers and vortex filters. Since this page is dedicated to biological filtration, please see the other parts of our site for information on each of these devices.

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