Alan’s Book

Chapter 6 Water Quality

The term “water quality” has several meanings; in the context of this book/chapter it refers to its suitability with respect to koi keeping.

Koi require certain water parameters before they can happily exist in our koi ponds and to this end several of the most important parameters are worth drawing attention to. Water quality in fish ponds is affected by the interactions of several chemical components as well as temperature.

General Information

Good water quality in a koi pond would typically have;

Oxygen levels above 6 mg/l

Ponds with aerated bottom drains ensure that the water column is continually exposed to the atmosphere at the surface of the pond where oxygen exchange takes place. This should ensure that good oxygen levels are maintained in the pond .Algal growth can affect oxygen levels during the daily cycle, especially is green water occurs.

Low levels of organic waste

Discharging the solid waste from bottom drains at regular intervals will control organic waste pollution and refresh the water by water changes.

Undetectable levels of ammonia when standard colourmetric test kits are used and in the order of 0.05 when using a digital meter.

Undetectable levels of nitrite when standard colourmetric test kits are used and in the order of 0.05 when using a digital meter.

Nitrate levels of less than 10 ppm

Water which has the correct pH and alkalinity value supplying an adequate filtration system which is correctly maintained should ensure very low ammonia, nitrite and nitrate levels.

Alkalinity (KH) values of 6-9dH (100-150 ppm)

G.H. values in the region of 6-11 dH (100 -200 ppm).

pH in the region of 7.8-8.1

All three of the above parameters can be manipulated where necessary. The alkalinity supports the pH and hardness values can be increased with lithaqua, calcium chloride and Magnesium (Epsom) salts.

Low levels of Nitrate

It is generally accepted that nitrate levels below 50 ppm are acceptable levels in koi ponds however there are some who report that levels as low as 10 ppm should be aimed for.


I have referred to optimum conditions for koi keeping and the optimum temperature range would be 14-24 C, varying slowly with the seasons.

Many koi keepers will find it financially difficult to maintain these ideal temperatures and in my experience some leeway can be tolerated with this particular water parameter.

Suitable heating systems utilizing electricity or a gas/oil boiler with thermostatic control of the pond water will ensure stable pond temperatures. Lower temperatures can be allowed for short simulated winter periods down to 8C when koi have been prepared for winter conditions by correct autumn feeding (see chapter 11 “Four Season Koi”). Any changes in water temperature are best done gradually; 0.5C to 1.0C changes daily are acceptable.

Low dissolved toxic metal content below the recommended safe levels for long term exposure.

The recommended safe levels of individual metals for long term exposure are given below.

Copper 5-15 ppb (Depending upon water hardness)

Lead 20 ppb

Zinc 30 ppb

Aluminium 50 ppb

Iron 30 ppb

Manganese 10 ppb

These values only apply when that one metal is present in the water. If there is more than one metal then the metals have a synergistic effect and the safety values should be lowered.

Serious koi keepers usually have their water tested for toxic metals and if necessary suitable filters can be included into the water supply to the pond.

Trace minerals.

Soft waters usually contain insufficient levels of trace elements and these are supplemented when lithaqua or oyster shells are incorporated into the filtration system. The addition of some types of clay provides minerals and trace elements too.

Chlorine levels below 0.03 ppm

When high chlorine/choramine levels are used by the local water authority a suitable filter such as activated charcoal can be included in the water supply to the pond. It is important to control chlorine in the supply tap water as intermittent high levels sometimes occur when Water Companies sterilise their supply pipelines. Chlorine filters incorporated into the water supply to the pond should be used to prevent disruption of the filter bacteria when chlorine levels are high (0.3-0.6 ppm)

Phosphate levels below 5 ppm to minimize problems with algae such as blanket weed.

Some interesting facts about water.

Carbon dioxide, pH, alkalinity and hardness are interrelated and can have profound effects on the levels of stress, and the toxicity of ammonia and certain metals in our koi ponds.

Temperature influences the density of water; it is heaviest at 4C which means that a natural pond with ice on the surface (0C) will have a temperature of 4C at the bottom of the pond. Ideally a koi pond should never be allowed to reach low temperatures where ice forms at the surface.

(In a koi pond which has circulating water, the top and bottom layers become mixed and the whole body of water tends to achieve the same temperature.)

Temperature also affects the oxygen concentration values of water too, having a maximum oxygen concentration of 14.64 mg/L at 0C, reducing to 8.25 mg/L at 25C and 7.82 at 28C.

At a comfortable temperature for koi, fresh water at 14C has a maximum oxygen concentration of 10.28 mg/L. These are the maximum values; in practical terms the quantity of oxygen available to our koi is less than the quantities given due to the natural processes which are occurring within the pond water such as bacterial nitrification and the reduction of other waste products.

These essential biological processes use some of the available oxygen which is available. Other inhabitants and processes also use oxygen which can be measured as “BOD”. This “biochemical oxygen demand” gives an indication of the quality of the water, samples with high BODs are usually due to a build up of detritus

The quoted figures for oxygen saturation can be distorted in some circumstances such as when algae are growing strongly in bright sunshine. At this time the oxygen produced by photosynthesis can elevate oxygen levels to 140% plus of the normal saturated levels for the given temperature. In ponds which are susceptible to the swings in oxygen levels (and pH) the oxygen levels (and pH) can fall to very low levels during the night.

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