The Dynamics of a Koi Pond - Basic Understanding of Water Quality

Submitted by Chris Neaves on Mon, 01/29/2018 - 15:21

  Jeff's Koi Pond, by Jeff Rasche

Water is a very remarkable substance. Without water life would not exist on earth. The extraordinary properties of water have a direct bearing and influence on the daily existence of our fish. A fishes body is composed of more or less 80% water. So it is easy to envisage fish as a volume of water (its body) separated from a volume of water (the pond) by a thin membrane (the skin). The most insignificant changes in the pond environment will therefore, have a direct and almost instantaneous influence on the life of our fish.

The pond it's self is literally a living entity. The factors that influence the water dynamics and parameters ebb and flow throughout the day as well as throughout the seasons. Each and every one of these Influencing Factors, let's call them - IF's - is dynamic. This is very important to remember when dealing with pond situations and perceived Koi problems. Sometimes we create problems which are not really there.

During the course of a 24 hour day each IF (Influencing Factor) has a natural dynamic change.

These dynamic variations can be recorded and a daily pulse or oscillation can be observed. As the seasons change, so do some of the parameters - the most obvious is temperature from the summer months to the winter months. A pond or water system has a natural bio-rhythm of life which fish have adapted to over many millions of years.

At any point of time, during the day or year, a measurement of one or the other IF will be different when compared to another measurement taken at a different time of the day or year. These fluctuations or dynamics should never be viewed in isolation. A single reading on a test kit will present a distorted view of what is actually going on in your pond. You may be panicking for nothing.


Before life can exist in water there must be oxygen in it. The pond must be activated with oxygen to begin with. This can be done via the tap water used to fill the pond or via the various devices we have on our ponds for circulating the water. Borehole or well water is lacking in oxygen and care must be taken to add air to this water before the fish are introduced. Oxygen is possibly the single most important element in pond water - and often a most neglected aspect of Koi keeping. For life to exist in water there must be oxygen dissolved in it. Whilst there are around 210,000 parts per million of oxygen in the atmosphere there are only around 8 parts per million in water - in a well aerated pond. A small increase in oxygen levels in pond water makes an enormous difference to the activity of the fish and other life forms. The fish, the bio-filter, organic decomposition etc. all extract oxygen from the system. Oxygen can only be introduced into the water through gaseous exchange between the atmosphere and water molecules. Oxygen is introduced into the pond water via continual contact with the atmosphere through design considerations such as drawing water from the further most area  from the atmosphere - bottom of the pond then returning it via as streams, waterfalls, fountains and apparatuses such as venturies. Air blowers can also be used. High turnover rates ensure a high oxygen level. Photosynthesis by algae during the day introduces addition levels of oxygen, but removes it at night. Air under pressure through venturies can cause gas imbalances in the water and need careful consideration in their placement in the pond. NEVER place them too deep.

Oxygen levels can fluctuate dramatically during the day and night as well as during periods of high and low temperature. There can be significant oxygen variations in ponds with poor circulation or in ponds which do not move the water away from the bottom of the pond. i.e. the point furthermost from the atmosphere. There is less oxygen in pond water at higher altitudes - at 1600m above sea level there is about 18 - 20% less than at the coast.

There is continual competition in the pond for the limited amount of oxygen available at any given time. The fish, the plants, the micro organisms all need oxygen rich water - all the time. Algae and submerged plants have a dramatic influence on oxygen levels in a pond during a 24 hour day / night cycle. The photosynthesis process during sunlight may rocket oxygen levels to saturation point and beyond. However, a dramatic plunge in oxygen with the reversal of the photosynthesis process at night can spell disaster, even to the point of fish suffocating in ponds at around dawn.

It has been found that if the oxygen levels are 25% below optimum levels first thing in the morning, growth rates are reduced. The turn-over rate will have a direct bearing on oxygen levels as will the stocking densities of fish. The faster the turnover rate the more water will come into contact with the atmosphere and the more gaseous exchange will take place. Stocking densities affect oxygen levels. The more fish in the pond, the less oxygen in the water, as they are all breathing all the time.

Higher temperatures in summer mean there is less oxygen that can be dissolved into the water. Higher temperatures result in faster metabolism, which in turn mean the less oxygen available is extracted faster and is needed in more quantities by the fish and other life forms than in colder temperatures.

A shortage of oxygen will be noticed by observing the Koi collection first thing in the morning. If the Koi are moving lethargically and hovering near the surface you may well have low oxygen levels in your pond. Oxygen shortages have been measured in ponds with fancy pumps and filters - but with a lack of exposing the water molecules to the atmosphere. A shortage of oxygen can also be observed by watching the breathing of the fish. If the fish are breathing heavily or "piping" they could be 1. Stressed or 2. Have a gill problem or 3. The pond water could be low in oxygen or 4. The fish could be exhausted for some reason. (and what have you been doing during the night to exhaust yourself, my little Cynthia Sanke?)

Keep in mind that the atmosphere is composed of only about 20% oxygen. Nitrogen makes up the bulk of the rest of the atmosphere. When using venturies i.e. pumping air into ponds under pressure, you are not only pumping in oxygen but nitrogen and the other gases found in the atmosphere. Nitrogen is more easily dissolvable into water than oxygen. High nitrogen levels will affect the behavior of your Koi. They will become lethargic and there will be some loss of appetite. You may also notice a discoloration of the white on the Koi in extreme cases.

Streams and waterfalls have a wonderful effect on the air pressures in the water and balance things out very nicely as they expose large volumes of the pond water to the atmosphere.


In the morning the pH will be low compared to a test done in the late afternoon. The buffering capacity of the water will play a major role in the dynamic fluctuation of pH during the course of a day. The pH you record may be unique to your pond. Don't be influenced by "perfect situations". It may be slightly alkaline with a pH of around 7.8 - 8.2. It may be that your particular pond has a pH range of 7.2 - 7.6. Or there may even be greater fluctuations. This is the natural, daily, pH cycle of your pond.

I strongly advise against attempting to adjust the pH to a perceived perfect figure of say 7.2. Measuring the pH in the late afternoon you will inevitably find it is on the high (alkaline) side. The addition of acid to pond water to lower the "high" pH can be disastrous at this point in time. The acid will lower the pH and so will the natural pond cycle during the night. The result - a pH crash - with the water becoming several hundred times more acidic in a few hours. As the buffering capacity of the water counteracts the acid a day or two later the pH climbs back to where it was before the addition of acid. All that was achieved was a radical swing in pH as opposed to a gradual natural dynamic change.

Sudden substantial fluctuations in pH values will stress Koi and possibly kill them. Although fish do adapt to changes they do not tolerate sudden, substantial changes in water quality very well. If they do not die, then the stress they suffer may become a contributing factor to lowered resistance and a possible disease infection. Measuring pH should be done in conjunction with measuring total alkalinity. The total alkalinity will give you an indication as to the buffering capacity of the water against pH variations.

The amount of algae (mostly free floating, single cell algae that makes the pond water green) can have a dramatic effect on pH, pushing the pH to very high levels in the evening after a full days photosynthesis. The opposite effect is observed by a lowering of the pH in the early morning.

The fish are not the only life in the pond. Not only the Koi but the toxicity of ammonia as well as the bacteria in the filters are affected by pH. The nitrifying bacteria that grow everywhere and in the filters thrive in alkaline water but do not function very well in acidic water. Should your water pH fluctuate through a narrow range, your water is well buffered. Should there be large fluctuations you may need to buffer the water a little. The pH should not rise much above 8.5 at its maximum reading.

More important than ideal pH water conditions is the fact that Koi have adapted to survive in a pH range of 6.5 - 9, providing the daily fluctuation within this pH range is not large and providing the pH change is not sudden. Nitrifying bacteria in our filters have an optimum pH range of around 7.8 - 8.5. Their upper limit is 9.4 and around 6 - 6.5 pH the nitrifying bacteria cease to function. Any adjustments to pH will have to be monitored daily and tended to on a daily basis.


On most occasions temperature will be lowest in the morning just before sunrise and the highest at sunset. (Unless your pond is built on a nuclear waste dump, which generates it own heat). Temperature is influenced by the amount of sun, the intensity of the sun the pond receives and other factors such as the wind speed during the day or night. Evaporation cools the water. Generally speaking Koi should not be subject to more than a 5ºC sudden change in water temperature. Always adjust the temperature gradually when moving Koi around. An upward movement is tolerated better than a downward movement in temperature by Koi. Most ponds have a 1 - 3ºC variation in temperature in 24 hour day / night cycle. Koi are more affected by temperature variations at lower water temperatures. And you will notice Koi are more affected by a sudden lowering of water temperature than a sudden rising of water temperature.

Certain areas in the country can experience a 10 - 15C atmospheric drop in temperature in a day accompanied by high winds. Ponds can and do drop temperature significantly in these circumstances. Hail storms are known to drop the temperature of ponds significantly. The Koi suffer from stress and disease problems can occur at a later date. In some cases the Koi catch a "cold". A white film is seen in the skin. The thin flesh around the head is the area where this is most easily visible. Should the temperatures rise within the next few days the "cold" will begin to cure its self. However, this may take some time to cure itself.

The Koi can be helped through large, sudden water temperature changes by the addition of a good grade coarse salt at a rate of 1 - 2 kg per 1000 liters immediately after the temperature has dropped.

Temperature has a direct bearing on the functioning of the immune system as well s the Koi's ability to recover from disease.


The levels of ammonia and toxicity of ammonia are never constant. A few hours after feeding the ammonia level will rise as the nitrogen wastes of the fish are excreted. This can actually be measured. A measurement just before feeding should produce the lowest or no ammonia reading (if everything is working). However, an hour or two after feeding a high ammonia spike will be recorded as the fish are now excreting their metabolic waste products. Interesting, a corresponding drop in oxygen levels at this time have also been measured in ponds as the Koi consume more oxygen for the metabolism of the food.

The higher the temperature the faster the metabolic rate and the quicker ammonia will be released into the surrounding water. However, there is another dynamic related to ammonia - the toxicity of ammonia (NH3) is not constant during the course of a single day. Ammonia is found in two forms in the pond. The toxicity of ammonia is pH dependant. Ammonia changes from ammonia to ammonium (NH4+) as pH drops i.e. it becomes less toxic. As pH rises (naturally during the day) the ammonium (or a percentage of ammonia) converts back to toxic ammonia.

Whilst very high pH above 9 makes a higher percentage of ammonia more toxic and has certain negative effects on Koi, low pH has other effects on the pond system. For example, the bacteria in the filter are oxygen and pH dependant.

So What Now?

Into this dynamic pond system we introduce two critical factors - fish and man. How can our Koi possibly survive in an environment that can become 100 - 200 times more alkaline / acidic, have a temperature change of more than 5°C, have oxygen levels that can drop and rise dramatically, and have a shifting ammonia / ammonium percentage in a single day? How can our fish possibly survive the bombardment of toxic chemicals which are added to the pond in the name of medications?

The average enthusiast does not have the finance to spend large sums of money on equipment to monitor the water continuously. How can he possibly monitor all these IF's (Influencing Factors) continuously, make daily adjustments and provide the perfect environment for his Koi?

The answer is simple - if your Koi are alive, healthy and growing well - you are doing it right.

Your Koi are the best test kits you have. Your observations of your collection are the most valuable part of maintaining the health of your collection. It may be a joke amongst Koi collectors that when a new pond is built they always suggest placing a few "pH testers" into the new pond to "test" the water. A joke yes - but it is an excellent way of testing the water for your new arrivals and new pond system.

The critical point to remember is that Koi are very capable of overcoming variations in their aquatic environment. Koi (coloured carp) have been living successfully in freshwater rivers for millions of years. There are fossilized remains of the direct descendants of carp. It is a scientific fact that whilst a pH of 7 - 7.5 is advocated as "ideal" for keeping Koi, Koi will live comfortably in a pH range of 6.5 - 8.5. A temperature range of 22°C - 28°C is ideal for keeping Koi. However, they winter well down to 10°C. And provided the temperature is not below 8°C for too long a period of time, they suffer no adverse affects after winter.

Oxygen levels are maintained by circulating the water correctly in the pond and by bringing the water into contact with the atmosphere continuously. The faster the turn-over rate of the whole system, the better.

Ammonia levels can be controlled successfully by providing additional surface area for bacterial growth. This extra surface area is often referred to as the bio-filter or bioconverter. The high oxygen levels provided by high turn-over rates will activate the bacteria and the metabolism of the fish.

The natural immune systems and osmo-regulation of the fishes body is adapted over millions of years to counteract natural fluctuations in the water and attacks from the outside world. Provided the fluctuations are not too great or too sudden the fish has the ability to adjust and survive the daily, monthly and annual bio-rhythms of the pond.



!If you're not havin' FUN, you're not doin' it right