Fish Disease Diagnosis
Fish Disease Treatments
Ponds, Aquaria & Filtration

Koi pond filtration and fish health

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.

Solid wastes

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. One of the only kinds of filters which positively allows you to remove wastes being held by the media are the new Aquadyne Bead filters so popular in America. (Bead Filtration)

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.

Filter types

There are two main types of external filter at use in the United Kingdom: 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 by most filter types almost impossible. Added to this is the inconvenience of having to constantly clean blocked foam pre-filters. By using Aquadyne bead filtration, cleaning foam filters is a thing of the past.

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.

"Koi Health & Disease" by Dr. Erik Johnson DVM is 200+ pages from Fish Health Practice - Readable by Beginners and delivered overnight from

Fish disease problems? Need some help? The help pages take you through the basics of the 'fish health work-up' diagnosis method and basic fish disease treatment methods.

90% of all health problems and fish deaths are caused by poor fish keeping! The key pages are essential reading for all beginners and intermediates who are serious about disease prevention.


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Expanded Content by Dr. Erik Johnson, and Used with Permission; Frank Prince-Iles ©2009 All Rights Reserved