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Koi pond filtration and fish health
As Koi-keepers we must control the concentrations of ammonia, nitrite and nitrate through proper care, filtration and water changes. Even though there will be some nitrification taking place in a Koi pond, the sheer volume and rate of ammonia production would soon devastate the pond and cause great harm to our fish if there wasn't some form of supporting water quality control.
Solid wastes
There are two major sources of ammonia - dissolved ammonia excreted by the fish and ammonia produced by decomposing solids. The pond filtration system must contend with both sources to diminish their impact on the fish. 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, regardless 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 controlling unwanted parasites and bacteria. How easily this can be achieved depends on the pond design and type of filter.
Filter types
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 from the 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.
Gravity-fed 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 is normally 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
Settlement chamber
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.
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