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KOI POND FILTRATION BASICS The heart of the Koi pond.

Although there will be some nitrification taking place everywhere in a koi pond, the sheer volume and rate of ammonia production would soon overwhelm the system and kill the fish if there wasn't some form of supporting water quality management. One of the main responsibilities of the koi-keeper is to control the concentrations of ammonia, nitrite and nitrate through good husbandry, filtration and water changes.

There are two main sources of ammonia - dissolved ammonia excreted by the fish and ammonia produced by decomposing solids. The filtration system should be designed to deal with both sources and to minimize their impact on the fish. If all solid waste could be removed as soon as it was produced this would reduce the source of a substantial amount of ammonia and other dissolved compounds. Unfortunately, this is not possible unless we mount a 24-hour guard with nets and pond-vacs - but we can design and maintain ponds in order to keep solid wastes to a minimum.

The important point to note is that solids should be removed from the system - not just hidden from view - otherwise they decompose and pollute the water, irrespective of whether they are located in the pond or filter.

Regular removal of solid wastes before they decompose is very helpful in reducing the levels of dissolved pollutants, improving water quality and discouraging opportunistic parasites and bacteria. The ease with which this can be achieved depends on the pond design and type of filter.


There are two main types of external filter: the pump-fed and the in-ground gravity-fed.

The cheaper, simpler method is to pump water up to a free-standing external filter. Having been pumped up, the water passes down through the filter medium and returns to the pond through a pipe or pipe work.

For very small ponds or those with few fish, this type of filter can be adequate but there are disadvantages for larger ponds and those with many fish.

Firstly, the 'dirty' water is drawn only from a relatively small area leaving most of the solids behind. A secondary form of cleaning, such as a pond- vac will be needed if the solids are to be removed efficiently.

Secondly, those solids that pass through the pump will be liquidized by the pump impeller, making efficient removal of solids almost impossible.

Added to this is the inconvenience of having to constantly clean blocked foam pre-filters.

The gravity-fed system works by pumping clean water from the end of the filter back into the pond, thus drawing "dirty" water into the first stage of the filter.

This can be a multi-chamber unit with a settlement section for the removal of solid wastes and a 'biological' section for the treatment of dissolved wastes.

It is more expensive and complicated to install but it is much more effective. Because water enters the filter by gravity rather than through a pump the solids do not get so broken up and can be removed more easily -assuming the settlement area is designed properly.

If the water feed to the filter is from a bottom drain and the bottom has sufficient slope, the water is drawn from a wide area, leaving fewer solids to settle on the bottom.

It is possible to design a 'self-cleaning' pond whereby virtually all the solid waste is removed from the bottom to the settlement area where it can be flushed to waste regularly. Obviously, any 'dead spots', where solids collect, may need regular cleaning with a siphon or pond- vac

The Illustration left shows such a system with a multibay filter plus vortex.

Although our choice with any sizeable system would not be a multi-bay set up the principle is much the same where water exits the pond through a bottom drain. (1) can be of aerated design (optional). From this point gravity takes over and the water will progress through the filter (2)-(3)-(4)From here it would be pumped back to the pond via the U.V.strerilizer. A final polishing filter if required could also be installed in this return line. In the illustration above we have shown a cartridge filter as an example.

There are two schools of thought as to whether the settlement chamber should be fed from bottom drain(s) or from mid-water. This really comes down to personal choice and depends how well the filtration system has been designed. Bottom-drain feeding is preferable because the solids are thereby continuously removed from the pond. The argument against bottom-drain feeds is that heavily contaminated water can enter the biological section of the filter and have an adverse effect on nitrification. Whether this happens depends on the design and type of settlement or pre-filtering. A large rectangular or cylindrical settlement chamber will allow most of the solids to settle out by gravity. If the water then passes through filter brushes or reticulated foam it will normally be clean enough to enter the biological section of the filter. A fairly slow flow rate through the settlement area is important if the suspended solids are to be removed efficiently.

Following on from above it can sometimes mean that vortexes on large heavily stocked ponds will need to be excessively large.

From this it can be argued that filters such as the Nexus make more sense having a much smaller footprint. They are also well designed for gravity systems.

The same urguement could be made in the use of bead filters with the obvious smaller footprint even taking into account the possible need for pre filtering screens (sieves) etc.

For more information please goto >>>>> Pond Filters

For more information please goto >>>>> Other Water Filters

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