Garden Pond Filters and Filtration

Filters and Filtration

If a garden pond is designed and built purely for plants and naturally occurring wildlife, a filter will not be necessary, as the biological systems will establish naturally, and the pond will develop at its own pace, without producing undue waste products. However, if the pond will be a home for fish, a filter should be seriously considered.



The main reason for filtering your pond is to clean up the environment. A pond is almost a self-contained ecosystem, and like any ecosystem, it needs a mechanism for removing the toxic waste. Since the garden pond is an artificial environment, it requires artificial methods to keep it free of waste. This enables you to keep reasonable numbers of fish in the pond. Before you can install a filter, however, you will need a suitable pump to force the water through the filter. The pump and water outlet positions should be considered carefully in relation to any waterlilies, which will not thrive in moving water. Where possible, an area of still water should be set aside for them.



There are three major kinds of filtration system: mechanical, chemical and biological. Mechanical filtration pushes water through a system incorporating gravel, clinker or foam, catching and removing all suspended particles as it does so. This is useful where there is a problem of plants or compost being stirred up, but it does nothing significant to improve the quality of the water. It simply cleans the water physically.

Garden Pond Filters and Filtration Chemical filtration removes toxic and undesirable chemicals from the water by using the absorbent properties of substances like carbon and zeolite. The latter is commonly employed in newly established aquatic systems to remove ammonia. It is a natural ion exchange resin, which replaces ammonia with sodium, so it is rechargeable with salt. Carbon can be used to remove chlorine, heavy metals, organic dyes and odours. Organic dyes, such as phenol and tannin, can be responsible for much of the brown discoloration in the pond, and they cannot be removed by mechanical filtration.

Chemical filtration is not a replacement for biological filtration; you will still need to provide a site for biological decay. If carbon is allowed to remain in the filter for a long period, it will become biologically active, which will cause a problem when it is removed, as the biological filtration capacity will be reduced, which will result in an increase of ammonia and nitrite.

It is possible to include both mechanical and chemical filters in the same unit, or they can be separate and connected to one another. Where two or more filter chambers are used, it is worthwhile fitting a by-pass pipe so that when one is replaced or cleaned, the other will continue to function. With new advances being made all the time, chemical filters are constantly being improved, and for the latest information, it is essential to visit your local aquatic supplier.

The most popular kind of filter is the biological type. This depends upon bacteria in the filter medium breaking down toxic substances, such as ammonia, into nitrites and then into nitrates that are readily utilized by the plants. This is quite a long-term project, for it takes some time for the beneficial bacteria to become sufficiently established to work effectively. Being part of the pond’s ecological system, albeit in a slightly artificial situation, they are also vulnerable to changes in water conditions. For example, they can be severely damaged by some pond cures intended to combat bacterial problems, for when the solution enters the filter, the beneficial organisms may also be attacked. It is worth checking whether this is likely to occur before buying such a remedy.

In a biological filtration system, some bacteria, which do not need oxygen, derive energy from being involved in a partial break-down of organic matter and, in doing so, release ammonia or ammonium compounds into the water. In confined conditions, these would be quite capable of killing fish, but other bacteria are also present and convert such harmful ammonia compounds into less noxious nitrites and nitrates, which are not all that harmful to fish and are readily assimilated by plants. These beneficial bacteria do require oxygen to survive and live happily in the oxygenated water that passes down through the filter from the upper layers.

Pure mechanical filtration, although limited in its ability to improve water quality, can be quickly activated. Merely switching on the pump ensures instant improvement. However, for long-term economy and success, the biological filtration method is preferable.



There are two main types of filter: internal and external. An internal filter has to be immersed in the water and, with careful planting, can be disguised. Ideally, a decision on its use should be made before the pool is constructed so that provision can be made in the excavation to incorporate it sympathetically, otherwise it may prove intrusive. For most pool owners this should not be a problem, for the modern portable internal filter is easily disguised and connects very neatly to the pump system. However, the filtration requirements of the koi keeper are likely to be significantly greater than the ability of even the newest, most efficient, portable internal filtration systems.

Of course, by employing an external filter, the pool will not be cluttered, but it does need to be in close proximity to the pool to function properly, and regular access for maintenance will be required. So an appropriate planting around the pool to disguise its presence is essential. If a rock garden is being incorporated as part of the overall feature, the filter system can often be hidden within it, though, of course, this will need to be planned from the outset.

For a filtration system to operate effectively, it should be connected to a pump with more capacity than the manufacturer recommends. It is always preferable to have spare pump capacity, despite the additional cost, rather than discover that a complete change to pumping arrangement proves necessary later on. However, the flow rate through the filter itself should match the manufacturer’.s specification. The size of pump is also critical if you want to incorporate a waterfall or fountain, for a much more powerful unit will be necessary if filtration is part of its role as well. However, in some circumstances, there are advantages to using a separate pump for each purpose.

Although apparently trouble-free, filters do require regular maintenance if they are to function efficiently. The frequency of maintenance will depend upon the kind of filter medium employed, together with the condition of the water, but a weekly check during the peak summer months will ensure that all continues safely.



The success of a filtration system is very dependent upon the kind of filter medium used. These are very diverse, and each has its advocates. Gravel is one of the most widely used, largely because it is readily available and comes in various grades, although most filters function best on a particle size of 6mm (1/4in). Some gravels have irregularly shaped grains and are of a porous nature. Their larger surface area providcs accommodation for bacteria, as well as producing good straining properties.

Processed lava granules are excellent, too. These are almost sponge-like, having a large surface area and a web of capillaries inside. This structure is a very efficient physical filtcr and also provides plenty of space for beneficial bacteria to colonize. It is inert, sterile, of neutral pH and also very light.

Baked clay granules are another medium that is sometimes used and, in fact, is favoured by a number of manufacturers. This material is similar to that used in hydroculture for the cultivation of house plants. It is clean and very easy to use, unlike sand, which is often recommended, but which in practice creates awful problems. The only role for sand is in a sand-fed filter, which is more the prerogative of the koi keeper.

Apart from natural and enhanced granular materials, some artificial filter mediums are very popular. Foam is among the favourites, although it does tend to clog frequently and require regular maintenance. Fortunately, it is very easy to clean. Only use foam that is recommended for the filter; do not try to economize by purchasing foam from a home-improvement store. The latter may be treated with a fire-retardant chemical which, when installed in the filter, could kill the fish.

Filter brushes are currently quite popular, too. These look like very large bottle brushes, being of cylindrical shape, and fit snugly into the system. Like foam, they need regular cleaning. In addition, they also provide an excellent home for bacteria.

High-surface-area scintered glass is becoming increasingly common as a filter medium, probably due to its ability to remove nitrate from the water.

The most bizarre filter medium is what are termed plastic mouldings. These are pieces of shaped plastic with large surface areas, which provide excellent homes for bacteria. The most common shape looks rather like a plastic spring or hair roller, the latter being a perfectly acceptable substitute.

Gravel is one of the most widely used filter materials as it is readily available and comes in various grades and sizes.

Baked clay and processed lava granules have a large surface area and a web of capillaries inside.

Foam is widely used, but it clogs easily and requires regular maintenance.

Only use special filter foam.

Filter brushes are very popular and are easy to clean. They fit neatly into a modern filtration system.

Scintered glass pellets provide a high surface area and are excellent for removing nitrate from the pool Shaped plastic mouldings have large surface areas and provide an excellent home for the all essential bacteria.



Filtering with plants is all the rage at present. Such a system can be adapted to the garden pool, too, and has long been practised by Chinese and Japanese fish breeders. It simply involves establishing a small subsidiary pond, or a filter box, with vigorous plants that will absorb harmful chemicals, such as phosphates and nitrates, from the water, then running that water through a conventional mechanical filter and back into the pool. The plants absorb all manner of excess mineral salts, in addition to nitrates ‘.and phosphates, all of which become locked up in their systems. Thinning the plants periodically removes the chemicals from the pond’.s life cycle. Unfortunately, using plants in this way will not remove significant quantities of ammonia and nitrite.



Most pond owners who decide to buy a filter would be well advised, initially, to purchase one that will suit their needs, rather than attempt to build one, unless they plan to keep large numbers of koi. Whatever configuration it takes, the ready-made filter will require careful placing, not only to function efficiently, but also to be as unobtrusive as possible. While a specially tailored filter would be easier to accommodate, until considerable experience has been gained, or expert advice sought, it is a route that is best avoided.

In addition to the mechanical and biological functions of a filter, there are two modes of operation: up-flow and down-flow. These refer to the route taken by the water as it passes through the filter medium. The up-flow system has a pipe that leads the water into the bottom of the filter chamber, where it permeates up through the filter medium. Conversely, the down-flow system permits the water to enter at the top of the chamber, then percolate through the filter medium to the outlet pipe and back into the pool. In a garden pool situation, there is little to choose between the two systems, although the down-flow type is easier to maintain, as the large debris collects at the top of the filter. Each would benefit from a pre-filtration chamber to remove large organic debris.

Apart from the box-style filter, it is also possible to introduce an under-gravel filter. This is an adaptation of the method widely used by aquarists, comprising a series of perforated pipes beneath a layer of gravel that is contained in some convenient manner, perhaps on a marginal shelf. It depends upon constantly moving water to keep the aerobic bacteria happy, allowing them to work on the organic debris and make it harmless. Such a filter must be made at home, and it is not the best option for a newcomer. Although it is quite efficient and easily disguised, it is difficult to maintain.



While filters function in various different ways, each has an outflow that discharges clean water back into the pond. During summer, it is beneficial for the pump to be some distance from the filter chamber so that water currents are created that mix the water zones and improve quality. To aid oxygenation, the water should also be returned to the pond above the surface. In winter, the pump can be moved closer to the filter and the surface of the water, the return being beneath the water to create a much smoother effect.


18. March 2011 by admin
Categories: Gardening Ideas, Water Gardening/Water Features | Tags: , , | Comments Off on Garden Pond Filters and Filtration


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