By D Griffiths 14.11.2001

Koi are termed Ammonotelic. Meaning most of their nitrogenous waste is excreted as Ammonia gas, rather than Uric Acid.
In a closed pond system this presents many problems to aquatic life forms but not least the koi.
Ammonia is primarily produced by the koi in the typical pond system and to a lesser degree by other by processes found in the pond system.( faeces etc) It is found in two forms free Ammonia NH3+ (un-ionised, free Ammonia) and Ammonium NH4+(ionised Ammonia), the former if which is considered the more toxic form.
Ammonia is dealt with in the pond system by a vast number of nitrifying bacteria in our biological filters; this process is termed the nitrogen cycle.
Ammonia NH3/4 is oxidised by nitrifying bacteria to become Nitrite No2, which as far as we as koi keepers are cocerned is just as toxic, but a second colony of Nitrifying bacteria oxidise this to the less toxic Nitrate NO3 roughly, about a hundred times less toxic than Ammonia.

As Ammonia is subject to lower ph levels below 7, it picks up a Proton and is ionised to form Ammonium NH4+, however if the ph rises again it is also capable of dropping the Proton to once again become Ammonia NH3+.
Most test kits available measure total Ammonia Nitrogen, which will be the combination of both forms,Ammonia and Ammonium.
The higher the ph of the water in the system, the greater the percentage of un-ionised, free Ammonia there will be and the lower the ph the more ionised Ammonium there is.
In a well set up mature filter/pond system, we should expect and only accept finds of very low or undetectable levels of Ammonia when we test for this substance, therefore it serves little purpose to test for which form of Ammonia you have, since if we also know the ponds ph we should have a greater understanding which is the predominant form of Ammonia there is NH3 or NH4.
Measurable amounts of Ammonia from a typical test kits indicates something is very much amiss and investigations should begin in earnest to determine the root course and remedial rectification should begin at once Anything but, un-measurable, low, Ammonia readings should not be tolerated in a mature pond. The general acceptable back ground level ammonia reading should be below 0.02mg/L

You would be tempted to think the koi was in harms way from any and all Ammonia in its surrounding water, and you would be right, I'm sure you have all smelt Ammonia in it rawest form (most commercially produced window cleaner etc) obnoxious is about the nicest thing you can say about Ammonia, it makes you eyes water and burns the nasal cavity and throat and takes your breath away. It may surprise you to know the exact same symptoms effect a koi, only the koi is a lot more sensitive to its effects, additionally, unless the keeper takes action, the koi cannot escape the effects of Ammonia and will be subject to burns to gills, skin, fins and the koi will be stressed beyond belief. But as the title of this paper insinuates, something else is going on of equally danger to the well being of the koi.

A koi eliminates waste nitrogen as Ammonia gas mainly via the gill lamellae. The koi's blood is very slightly acidic, and there are reason for this phenomena, but the ammonia produced by the koi will be in the form of NH4+, ammonium. This process is termed Osmosis and diffusion.
People often think of Osmosis and Diffusion in terms of body fluids and salt, moving from one side of the koi to the other via the gill but in fact almost anything dissolved into fluid /water can migrate via the osmotic/diffusion process provided the molecule is not to large to pass through the holes in the membraneit's traveling across,

(Osmosis is when a fluid moves across a membrane from a high to a lower pressure to equalize.)
Osmosis ceases to happen when the two concentrations each side of the membrane achieve equal pressure,

Diffusion is when a dissolved substance moves across a membrane to equalize the concentration; this is termed an Isotonic state.
A saline concentration that is higher outside the koi to its internal concentration is termed Hypertonic state.
A lower salineconcentration outside the koi is termed Hypotonic state. A Hypotonic and Hypertonic state will try to achieve Isotonic status, provided the membrane is at least semi-permeable and the molecule involved is not too large to pass through.

The method of waste removal (Osmoregulation) presents certain inherent problems to the koi. If you subscribe to the theory that the majority of Ammonia in the pond is being produced as a by-product from hungry koi via the gill( and this will be the case; unless something is sadly amiss with the pond / filtration system), it follow that the koi is the major source and in order for the koi to rid itself of this toxin the target for the unwanted Ammonia is the pond water.
Unlike Nitrite (NO2) Ammonia NH3/4 the source is in the main the fish and the pond water the target, Nitrite NO2 however the source is the pond and the fish is the target.
With the process of osmosis/diffusion, the target for Nitrite has to be the internal body fluid of the koi via the gill lamellae because the pond is always the higher cocentration but with ammonia its the reverse.
So you could say in the *natural* sequence of events in a typical pond is, the Ammonia is trying to get out of the fish into the pond and Nitrite is trying to get into the fish from the pond.

Most folks think of running high Ammonia levels in the same light as Nitrite, in so much as its perceived as physically damaging the fish from the outside also to be attempting to enter the fish, and once there will do still more serious damage the fish. The answer to this one is YES and NO,
Yes the ammonia will be doing sever damage, irritation and stress to the koi, which in itself has serious enough consequences. But the Ammonia is not trying to get inside the koi, because the koi is the source of the Ammonia and the physics attached to the rules of diffusion dictates that as the Ammonia level rises in the koi's tissue fluids, it is trying to get out of the koi and the best case scenario for the koi is equal concentrations inside to outside.
example, if the pond water is running at a level of Ammonia @1mg/L the way diffusion works, it will be a fact that the level of Ammonia in the koi's fluid will be around the same because the koi is the source and can only release it to equal concentrations, So it follows that if the ambient Ammonia in the pond is almost zero the koi can release Ammonia to that self same level, and if for some reason the filter is new and immature or has stalled or simply can't cope with the organic load placed upon it, as the koi releases still more Ammonia into the water and the Ammonia level rises in the pond and the filter is not clearing it, so the level of Ammonia *IN* the koi rises to match the external concentration.

If it helps, try to visualize it as two empty containers side by side each chamber connected at the bottom by a tube with a fine membrane across the tube, separating the two chambers, pour a pint of water (which represents the koi's body fluid) into one of the containers, (this container will represent the koi), this will increase the pressure in the one chamber. As soon as it can, that pint will displace into the other container (which is representing the pond), you end up with equal measures/pressures in each container. We then add still more body fluid, (another pint of water) and end up with one pint in each container. And the membrane is called the pressure point. This is Osmosis.

Still with the pint of water still in each chamber dissolve two ounces of salt into the side representing the koi, and over a period of time without moving any water the salt will diffuse across the membrane to the lower concentration or into the non saline chamber, till you end up with the equivalent of 1 ounce per pint in each side
This is the Diffusion.

In reality both the above work at the same time as the salt moves one way the water moves the other, this is termed,Osmoregulation


So you can see from the diffusion example if we substitute ammonia for the salt, that as the koi is the source of Ammonia, it can never be lower in Ammonia content than the concentration of the pond. This is the principle of Diffusioni.
You may now be forming a differing opinion as to if it's the external Ammonia or the internal Ammonia that will eventually kill the koi.
If Ammonia levels are allowed to run continually high in the pond, I think the true answer would be more of the internal but not excluding the external level of Ammonia that will also contribute its fair share of damage.

This may help explain the anomaly of some claims that, when a koi is being transported in shipping bags over long distances from Japan, where by the koi dump huge amounts of Ammonia into a limited unfiltered amount of water in the transport bag. Some people claim the koi is able to reabsorb Ammonia from the water as a safety valve to keep it from harm, and will dump this accumulated Ammonia at the first opportunity when conditions are correct for it to do so.
Under the principles of Diffusion it's just not possible for a koi to reabsorb Ammonia from the bag water as it is always trying to add to it, in fact, as the Ammonia level in the koi continually builds up to levels above the outside concentration the koi will add to it, but not to the point were the outside is higher, the two levels internal and external levels will rise steadily together.
It is true that a koi, when conditions are right, i.e., in an non polluted pond the koi will dump this excess accumulated Ammonia post haste, but this is not the act of reabsorbing Ammonia, but rather retention of Ammonia. Two very different things.

However there are conditions where a reverse of the osmoregulation system can happen. If something else is adding ammonia to the pond system faster than the koi can (i.e. rotting vegetation, algae, food, etc, this could lead to I higher concentration of ammonia being outside the koi and in this instance the ammonia would reverse its way into the koi being as the koi is now the lower concentration.
The above gives a clear explanation of how a koi rids itself of Ammonia but the koi is as much at risk from the accumulating amounts of Ammonia toxicity internally as outside with high ambient Ammonia readings.

Therefore the koi is at risk from the enemy within.