Ammonia at High pH levels is Dangerous to Pond Fish in Small Concentrations.

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I am pretty confident that most pond keepers have come across warnings about ammonia in pond water. Certainly those koi enthusiasts who are brave enough to "show" their fish are acutely aware of this problem and they go to great lengths to prevent disasters taking place.

Next time you go to a koi show you will see water is always being emptied and refilled into various holding tanks. This is done to ensure ammonia levels never reach toxic concentrations.

This article delves a little more deeply into ammonia, what it is in a pond environment and under what circumstances it is really dangerous. By following this short article you will be well able to identify and handle potentially dangerous conditions developing in your pond.


Important new article for Pond Keepers facing Spring-time...

Many things happen in a pond as the water temperature increases. This starts to happen as winter ends and spring starts. Don't get caught out. Get your pond ready

Persistent and high ammonia concentrations can only be handled in one way: by changing large portions of pond water on an ongoing basis until the cause and cure of the problem has been identified and rectified. Also STOP feeding the fish. However there is no need for any fish pond to get into a situation where high ammonia levels are a problem so long as an effective biofilter is installed.


Ammonia comes from the food we feed our fish

  • All fish and humans for that matter need proteins in order to grow big and strong. All proteins contain amino acids which all contain nitrogen which becomes the source of the ammonia we get in all fish ponds. The trouble with ammonia is that it is poisonous to fish. This sounds crazy - a fish eats food to grow but in doing so sows the seeds for its own death by poisoning.

  • Of course this is not what happens in nature because the levels of ammonia do not build up to poisonous levels. This is achieved in a lake for example by restricting food supplies, limiting fish population and ensuring there is a biological balance that prevents ammonia building up.

  • In a pond however ammonia build-up can be dramatic and deadly. To understand what can happen and to therefore prevent dire consequences a basic understanding of ammonia chemistry in water helps.

If ammonia is present in water (this means every pond with fish and other living creatures in it) it can be present in 2 forms ....

  1. Ammonia which is deadly or

  2. Ammonium which tends to be a far less harmful

Ammonium is continuously transforming itself into ammonia and hydrogen ions and vice versa. In any situation this transformation reaches an equilibrium or balanced state. Changing the situation by changing pH or temperature for example will disturb the old equilibrium and create a new one. Unfortunately some equilibrium conditions are far worse than others as we can see below.

  • At a pH of 7 or less only ammonium is present (remember ammonium is far less poisonous than ammonia but still poisonous).

  • At a pH of about 8 around 5% is ammonia and 95% ammonium

  • At a pH of 9 then 50% is ammonia and 50% is ammonium. At this point we are heading for big trouble.

  • At a pH of about 11 there is no ammonium only ammonia. Your fish are also all dead.

The sad point is that there does not have to be a great deal of total ammonia to create havoc in a pond and the higher the temperature of the water the less there needs to be even when the pH is constant.

I came across the following table on the internet which shows clearly the potential problem in an aquarium. In the table below the figures relate to the maximum long term level of ammonium or ammonia in mg/litre or parts per million (ppm) depending on pH as discussed above.

  Water temperature degrees C


20 deg

25 deg
















This table indicates what was said above namely as pH climbs then hardly any ammonia can be tolerated if fish health is to be the prime consideration. In practice every pond keeper must strive to obtain no measurable ammonia and/or ammonium in the pond on an ongoing basis.

At this stage I would like to point out that pH levels of 9 are not uncommon in ponds when there is a large amount of algae present and the water lacks what is called alkalinity. This is the subject of another article on water quality. Suffice to indicate large algae blooms are potentially dangerous to fish because they can cause sudden and severe increases in pH. This is another reason for installing an UV light even in the best koi ponds with excellent biofiltration.

Koi food becomes koi poison

Gerry Preston who writes superb articles on koi keeping in the UK magazine Nishikigoi International calculated that between 3 and 4% of the dry food fed to koi becomes ammonia which is secreted by the koi into the pond water. Gerry also showed that immediately after feeding ammonia levels tend to be at their maximum. In addition he also showed that it is better to feed little and often rather than a lot at once in view of this "spiking" of ammonia levels.

All however is not doom and gloom (just that it can be and this is what I have wanted to show in this article). The solution to this potentially disastrous situation of ammonia poisoning is the installation of a well designed biofilter big enough to cope with the existing fish density and also the future stock density that will arise from the fish growing.

To finish off the article it is worth pointing out that the nitrogen contained in the ammonia actually sticks around in the pond water and is quite difficult to get rid of. An installed biofilter however creates a situation whereby the ammonia is transformed first into nitrites and then into nitrates. This series of reactions is referred to as the nitrification cycle. Nitrification can only take place in the presence of specific bacteria accompanied by large amounts of oxygen. More about this in later articles.

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