The Elements of Water Quality

Submitted by Chris Neaves on Mon, 01/29/2018 - 15:28

The critical water parameters in Koi keeping are:

  • dissolved oxygen
  • temperature
  • pH
  • ammonia
  • suspended solids
  • nitrite
  • alkalinity
  • carbon di-oxide.

Each water quality parameter interacts with and influences other parameters, sometimes in complex ways.

Concentrations of any one parameter that would be harmless in one situation can be toxic in another.

For example when aeration and degassing problems occur, carbon di-oxide levels generally become high, while at the same time dissolved oxygen levels become low. The result of this particular situation is that only is there less oxygen available to the fish the fish are less able to use the oxygen that is available. The high carbon di-oxide level of the water affects the fishes blood capacity to transport oxygen aggravating the stress imposed by low dissolved oxygen levels.

Another excellent example of the complex interaction among water quality parameters is the relationship between pH and toxicity of ammonia. Only the unionized fraction of the total ammonia concentration is toxic. At a low pH most of the ammonia in the water is in the non-toxic ionized form. However increasing pH by only one unit from 6.5 – 7.5 pH increases the concentration of toxic ammonia by a factor of 10.

Simply adding baking soda to a system to increase alkalinity can inadvertently increase the unionized ammonia to toxic levels.

The relationship between water quality parameters and their effect on fish growth and health is complicated. Fish lack the means to control their body temperature – it is entirely dependent on the environment. Environmental temperature changes affect the fishes rate of biochemical reactions, which leads to a different metabolic and oxygen consumption rates.

At low temperatures metabolic rates decrease, As temperatures increase koi become more active and consume more oxygen but more carbon-di-oxide is produced as well as other excretory things such as ammonia.

Water has several unique properties.


Temperature affects the whole pond system as well as the metabolic activity of koi. Water temperature has a direct effect on respiration rates, growth rates and feeding rates. The optimum water temperature for koi is above 20⁰C. However, above 30⁰C there is less oxygen in the water and conversion rates of feed are reduced. Below about 10⁰C metabolism slow down dramatically and consequently the appetite for food becomes less. However, as koi are still utilizing energy to breath and swim at low temperatures some nutrition is necessary.

Not only the koi are affected by water temperature so to are the nitrifying bacteria. Nitrifying bacteria prefer a temperature range of 25 - 30⁰C. Growth rate is decreased by about 50% at 18⁰C and growth rates are decreased by about 75% below 10⁰C. There is no nitrifying bacteria activity below about 4⁰C. Nitrifying bacteria begin to will die at around 0⁰C and over 49⁰C. Nitrobacter is less tolerant of low temperatures than Nitrosomonas, therefore at colder temperatures monitoring the build-up of nitrites is a good idea.


Effects of photosynthesis on pH. During day light aquatic plants remove carbon di-oxide from the water for use in photosynthesis. Both plants and animals are continuously releasing carbon di-oxide into the water by respiration. During daylight aquatic plants usually remove carbon di-oxide faster than it can be replaced by respiration. As carbon di-oxide is removed, carbonate accumulates and hydrolyzes and so the pH increases. Plants continue to use the small quantity of carbon dioxide available at pH values above about 8.3 and bicarbonate may be absorbed by plants and some of the carbon from bicarbonate used in photosynthesis. pH will rise during day light hours – especially ponds with a lot of algae such as suspended algae which make the pond pea green

When pond water is poorly buffered and bicarbonate concentrations are low pH values can rise to 9 or 10 during periods of intense photosynthesis (day light hours). In warm months in eutrophic water the conversion of carbon dioxide to organic carbon by photosynthesis may exceed the release of carbon dioxide from organic carbon by respiration so in the early morning pH values are at their lowest.

It is the 24 hour cycle or swing in pH that is important as a large fluctuation in pH is very stressful to koi.

pH also has a direct bearing on the functioning of nitrifying bacteria. Generally speaking nitrifying bacteria prefer an alkaline environment with Nitrosomonas functionally optimally between 7.8 - 8.4. The optimum pH range for Nitrobacter is slightly less at pH 7.4 - 7.8

Nitrifying bacterial growth is inhibited at a pH of 6.5 and all nitrification is inhibited if the pH drops to 6.0 or less. Care must be taken to monitor ammonia if the pH begins to drop close to 6.5. At this pH almost all of the ammonia present in the water will be in the mildly toxic, ionized NH3+ state.

Dissolved Oxygen

Should always be as high as possible and is possibly the most important water quality factor. Of all the water quality parameters, dissolved oxygen is the most important as well as being the most critical parameter. Dissolved oxygen levels are highest at low temperatures and lowest at high temperatures – the exact opposite of what is needed!

Koi spend a vast amount of energy removing dissolved form the pond water compared to land animals. As salinity increase so the dissolved oxygen levels decrease. So to with altitude – the higher the altitude above sea level the less oxygen can be dissolved into pond water. On the high veldt there is about 18% less oxygen in pond water than at the coast.

Koi feed best, grow fastest and are healthiest in warmer water and with oxygen levels above 5 – 6 mg/L. The gills can only transfer so much oxygen to the blood and attempting to get saturation or higher levels in pond water


  1. Ammonia -  Should always be as low as possible. <0.01mg/L or ppm.
  1. Nitrite - Always be as low as possible. < 1.0 mg/L or ppm or < 0.1 mg/L in soft water.
  1. Suspended Solids  - No more than 10 – 80 mg/L
  1. Alkalinity  - Should always be between 50 – 300 mg/L. Calcium and magnesium are the major sources of alkalinity in water.Soft water id considered to be from 0 – 75 mg/liter. Moderately hard water is considered to be 75 – 150mg/liter. Hard water is considered to be 150 – 300 mg/liter and very hard water is considered to be above 300 mg/liter.
  1. Carbon di-Oxide  - Should always be under 60 mg/L

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