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Water quality

Consequences of overcrowding

Recently a very interesting discussion took place on the Koi4u forum. The discussion centred on the consequences of overstocking and it turned out as a highly emotional topic. The fact remains that the koi keeper creates a far from perfect environment for koi that can never accurately duplicate the natural environment, unless some new technology will enable us to do so. Because water is not an unlimited resource, ponds have to be constructed as a closed system where water is recycled. The hobbyist today can choose between a myriad of filter systems, but in essence the principal behind the systems remain the same. With the exception of perhaps ozone, all utilise bacteria to do the work with varying levels of success. It is also not possible to devise a generic formula to determine the correct stocking levels for all ponds. There are just too many variables that come into play in ponds because of the differences in water volume, surface area, filter system, aeration, turnover rate, feeding regime, water source, water changes, food quality and maintenance schedule by the owner.

To argue about these things will not lead to a solution, because the motivation behind keeping a koi pond may vary from person to person. The motivation may vary between the keepers that buy the best genetically refined koi and aims to raise it to win major show awards, to the keeper that buy commercial grade koi that is kept for enjoyment and the only concern is the health of the fish. All koi keepers will fit within the above parameters and for all of them, overcrowding will mean something different.   It is therefore a very "subjective" topic because where do you draw the line? One person may say that the correct stocking level is 500 litres per kg of fish because the water parameters in his pond have stayed within limits and there were no ulcers or fatalities in three years. Another person may counter by asking the growth rate of fish in that pond and the quality of the colour plates.   In the light of the above one can only look at the consequences of "overcrowding" if you can understand the effects that it will have on the fish and then position yourself to satisfy your own koi keeping needs.

It must also be pointed out from the outset that various factors can come into play and these factors will have an influence as a combination of factors or individually on the koi health, koi growth or koi development.   The following factors are always at play in a pond and will have some consequences to the fish.   Number and size of the fish Please visualize a "Jumbo" koi as a large bag. Then visualise how many Tosai will fit into the "bag". You will be surprised to find that there may fit 20 to 30 tosai into the same space as the one Jumbo fish. Now consider the fact that a 10 cm fish has the same number of filaments in its gills as a Jumbo koi. The only difference is that the surface areas on the gills are in proportion to the body weight, much higher for a small fish. Small fish can therefore survive much longer in oxygen-depleted water. This explains the reason why the big fish succumb first when there is a power or pump failure. It is therefore obvious that the needs of a Jumbo koi are vastly different than Tosai. If a pond can easily house 30 tosai, it will be barely sufficient for a single jumbo koi.

Water quality

Water quality is the most important single factor that will influence the development and health in a koi collection. Persistent low levels of ammonia and nitrite or high levels of Nitrate will stress fish over time and the effects of stress is normally reduced growth, contraction of the pigment cells (chromataphores) and eventually the destruction of the whole colour plate. It is also common knowledge that stress has a marked effect on the immune system of koi. Please note, in situations where a pond is overstocked, there will always be low levels of ammonia present in the water, regardless of the turnover rate. It should therefore be expected that in ponds with a high fish stocking density, a great demand will be placed on the buffer system, resulting in frequent pH swings. The release of hormones and enzymes by fish play a huge role in koi growth and development, similar to the release of substances in a colony of ants. New queens will only hatch if the excretions of certain chemicals are reduced. The release of hormones and enzymes is nature's way of inhibiting/culling of overpopulation in a specific water body. In our ponds where we recycle the water, there is a continuous build-up of these substances. Unfortunately the best fish are genetically the furthest removed from the wild carp and are the first to suffer. This may include suppression of the immune system, slower growth and over time the outbreak of disease.

If there is a continuous sufficient replacement of water, the size of the pond is not really relevant. Koi do not measure the size of the pond and then decide to stop growing. The determining factor is the excretion and increase of certain enzymes and hormones in the water. The larger the pond and the lower the stocking rate in a closed system, the lesser the chance of a hormone and enzyme build-up and therefore it has a lesser influence on the koi. 

Composition of koi population in a pond.

It is a general accepted rule that it is best to keep fish of similar size in a pond. The most compelling reason is the difference in feeding habit between small fish and larger fish. Larger fish enjoy eating at leisure and may take up to 30 minutes to consume the required amount of food. Young fish especially Tosai and to a lesser extent Nissai will frantically dart around and eat the required amount of food in minutes. The above scenario leads mostly to a situation where the large fish gets "irritated" and withdraw from feeding or at best do not get to the food in time before it is snatched away by the smaller fish.  In this instance the situation is to the detriment of the large fish. There may be a situation where this behaviour is not applicable for instance where there are not sufficient small fish to bolster their courage. A single small fish rarely have enough courage to disrupt the feeding behaviour of large fish, and the situation is reversed. I can just ad that I believe there may be an advantage for the smaller koi during feeding times, because of the vastly different behaviour of a young koi and that of a more mature koi.


There is an old saying amongst fishermen: "an old fish is a clever fish" In the wild, young fish will school in their thousands and competition is rife. The small fish will risk their lives to get food. Five years later the school of fish has been reduced for various reasons to only a few adults. The behaviour is totally different. The stocking density will also be influenced by the quality of the food, the amount of food and of course the frequency of feeding. All these factors will influence the pollution and the amount if fish the pond can sustain. 

Water parameters

Some will say that if the water parameters stay stable, the stocking rate is fine. I believe there are more factors to consider than just water parameters as discussed above. Dissolved organic content Fish waste, sloughed off slime, leaves and dust contributes to suspended organics and therefore it increases the oxygen demand in a pond. In a heavily populated pond, the mentioned organic material leads to greater acidification. Pond design The pond design also plays a major role in the well-being of the koi. The circulation and rotation of water will determine if there are "dead spots" in the pond. Water in these "dead spots" normally contains less oxygen and more waste matter. This is an ideal place for pathogens to flourish and effectively makes the water volume in which the fish population can move around in, much smaller. Less fish can be housed in a pond where dead spots occur. Another factor that will determine the stocking density of a pond is the surface area in relation to the depth of the pond. In a deep pond with a small surface area the amount of water in contact with the air is restricted and oxygen cannot enter the water in sufficient amounts unless the bottom water in the pond is "lifted" to be in contact with air on a continuous basis. The incorporation of a waterfall into the design of the pond may assist with gas-exchange.

The last factor to remember is that a protruding object in the water is like a magnet. Sooner or later a fish will hurt itself whether it is during a feeding frenzy or during normal activity. Any rocks, pots or other ornaments in the pond will actually reduce the water volume of the pond. The pond design will also determine the location of the pond. The location can become a nightmare for the owner. It may be built where there are too many deciduous trees, too much sunlight, and runoff water from the roof. Such water may contain pollution that will be washed into the pond, causing innumerable problems. Secondly it can be build where leaves, dust and also disturbances and predators will occur. The pond design may further influence the temperature changes that will be experienced by the koi. The factors that may influence temperature changes are location, depth, size as well as whether the pond was constructed above or below ground.   Frequent large temperature changes will add to the stress of the koi in that pond.  

Filter system

The effectiveness of a pond's filtration system will to a great extent determine the safe stocking rate.  The filter system should be able to remove solids as well as convert ammonia and nitrite very effectively. Without an effective filter system the stoking rate should be the same as the stocking rate of a mud pond. Obviously such a stoking rate will not suit the hobbyist. Even if the filter can remove all ammonia and nitrite from the water that is pumped through it, the turnover rate should be high enough remove the continuous excretion out of the water before there is a high enough level of ammonia or nitrite to harm the fish. At high stocking levels the turnover rate should be very high. Pond husbandry will also play a major role in determining the stocking level of a pond. Removal of accumulated debris in the settlement chambers and filters goes hand in hand with water changes. Water changes dilute protein, hormones, enzymes and nitrates as well as beefing up the buffer system   Feeding regime I can just ad that I believe there may be an advantage for the smaller koi during feeding times, because of the vastly different behaviour of a young koi and that of a more mature koi.

There is an old saying amongst fishermen: "an old fish is a clever fish" In the wild, young fish will school in their thousands and competition is rife. The small fish will risk their lives to get food. Five years later the school of fish has been reduced for various reasons to only a few adults. The behaviour is totally different. The stocking density will also be influenced by the quality of the food, the amount of food and of course the frequency. All these factors will influence the pollution and the amount if fish the pond can sustain.


Now to come to the survival of pathogens, and in this instance I refer mostly to parasites. 

If one look at the life cycle of parasites, the first thing that comes to mind is the parasites that multiply through binary fusion. These parasites, although they can swim, mostly spend their life on one host and reproduce there.

Others will lay eggs and as juveniles they are in a race against time to find a host. Some of them have remarkable ways of finding one.

The last group of parasites go through a series of stages before they are actively looking for a host.

In a very sparsely populated pond the chances of a juvenile parasite to find a host is slimmer than in an overcrowded pond. That is the reason why ulcers and parasite infestations occurs more frequently in overpopulated ponds. The parasites find hosts easier so the survival rates amongst these parasites are high. There is also frequent contact between fish in more crowded ponds and parasites can be transferred from one host to another.


The management of stress in a koi collection should never be underestimated.

Some of the stressors that occur in a pond are too much handling, frequent visits by predators, badly formulated or old food, temperature fluctuation, unstable pH and overcrowding. In fact, all the above will contribute to stress to a certain extent. However, the most important stressor is bad water quality (see article on water quality). Ammonia, nitrite, pH, low dissolved oxygen, nitrate, dissolved gasses and unstable temperature play a vital role in the functioning of the fish's defense systems. The above influences can actually weaken or shut the defense system down, with disastrous results. In most cases disease outbreaks or unexpected mortalities can be traced back to some chronic stressor in a pond. In other cases, it can be traced back to a highly stressful situation that occurred a few weeks prior to the symptoms occurring. To cover this subject in full may require a book the size of an encyclopedia, and then months to convince all hobbyists.

For this discussion, the effects of overcrowding should be considered. If a person has 40 medium sized fish in a pond of 20 000 litres (which I see often) and he confirms that the ammonia reading is zero, I am highly skeptical. Fish excrete ammonia continuously and if the turnover rate is not once every few minutes, there must always be ammonia present in the pond. A continuous low level of ammonia causes stress in fish. Such a stocking level will also cause pH swings.

Unless there is a small Niagra Waterfall, there will be a relative low oxygen level and not enough exchange of dissolved gasses. Fish waste, proteins and slouched off slime will increase the dissolved organic content in the pond, lowering the oxygen further. Combine this with competition for food, excretion of hormones and enzymes and the odd medication and you have a cocktail that will increase the stress in the collection to an unbearable level. Under such conditions bacteria blooms and parasite infections are a real danger.

The above is just to indicate the health risks involved in overcrowding. In this example, there is a continuous fight for survival, let alone proper development of a fish to its full genetic potential.
In my opinion it is highly unlikely that one can give a generic formula for stocking densities, but regardless of the efficiency of the filter system, the lower the density, the less health problems one will experience and the better the development of the koi. 

Last Updated on Sunday, 20 November 2011 08:56