Koi Pond Chemistry

As koi keepers we are more water keepers than biologists, and as water consist of H2O (as well as everything else dissolved within it) we owe it to our koi to understand a little chemistry. This will then increase our success and fulfilment, ultimately benefiting our koi.

Water is an amazing substance that makes life on earth possible. Besides making a pond ‘wet’ and providing something for our koi to swim in,  water enables all the necessary chemical reactions to take place that help maintain a pond’s health and balance.

Water is the world’s best solvent. You can tell where water has been by analysing what it has dissolved in it. For this reason, the water in an upland, mountain stream is relatively low in dissolved minerals, whereas the opposite is true for a lowland river or lake. A koi pond is best modelled on the chemistry and water quality found in a lowland lake as this is the natural habitat of the wild-type ancestors of our koi.

Water and ions

A molecule of pure water (H2O) consists of 2 hydrogen atoms joined to one oxygen atom. In a pond, the water molecules separate (or dissociate) into ions – positively charged hydrogen ions (H+) and negatively charged hydroxyl ions (OH-). These ions are constantly forming and reforming and will do so with any ion of an equal and opposite charge. It is this characteristic of water that makes it such an excellent solvent. For example, common salt – sodium chloride (NaCl) will dissolve readily in water, forming sodium ions (Na+) and chloride ions (Cl-). This is true for many other elements and compounds, meaning that each aquatic environment in the world, will have its own unique chemistry.


pH is a measure of the acidity or alkalinity of a substance and is an important measure for pond water. It is measured on a scale of 0 to 14, where 7 is neutral, below 7 is acidic and above 7 is alkaline. Pure water is neutral (neither acid or alkaline) and is the standard against which acidity and alkalinity is measured. pH actually measures the quantity of free hydrogen ions (the ‘H’ in pH) and recognises that free hydrogen ions make a substance acidic. More hydrogen ions relative to hydroxyl ions will make the pond acidic, more hydroxyl ions relative to hydrogen ions will make the pond more alkaline – so pH is a matter of balance. Taking lowland lakes as our pond’s model for water quality, our pond’s pH must be alkaline – being stable between 7.0 and 8.5.

Why do koi prefer slightly alkaline water?

As descendents of carp, our koi are a lowland, still water fish. By the time fresh, clean and relatively pure mountain water has reached their natural habitats, it would have picked up an array of salts and minerals, making the lowland waters alkaline and hard. As their physiology has become adapted to this water chemistry, they must be provided with a similar pH and water quality in ponds or they will become stressed. The target pH is between 7.0 and 8.5.

Processes in a pond that can affect its pH.

As pond keepers, we should aim for two things when managing our pond chemistry.

1. A water quality that is within our koi’s natural limits (ie what their physiology is adapted to and able to tolerate)

2. Having achieved a suitable water quality, that it remains stable and within our fishes’ natural limits to avoid stress and disease.

This especially applies to pH. There is a constant tug-of-war occurring in a koi pond where various natural factors conspire to alter the pH with a tendency to cause fish problems. Essentially there are 3 problems that you may need to address in your pond.

1. The pH is consistently too low

2. The pH is consistently too high

3. The pH is very unstable and prone to wild fluctuations.

What if pH is less than 7.0 and becomes acidic?

In effect, speaking chemically, this means that there is an abundance of free H+  ions. A number of natural biological processes will put pressure on the pH of a koi pond to become acidic.

a. Biological Filtration. When koi release ammonia (NH3) they are releasing nitrogen and hydrogen ions. When that ammonia is broken down by bacteria into nitrite (NO2) the 3 free hydrogen ions are released into the water, causing a drop in pH. As ammonia excretion and biofiltration proceed unabated in every pond, there is a relentless downward pressure on pH in every pond. If the pH is allowed to drop below 7, then the addition of further free hydrogen ions accelerates the drop in pH, leading to a pH crash, causing your fish and other aquatic life real stress.

b. Respiration. Plants (including blanket weed), fish and bacteria respire constantly taking in oxygen and releasing carbon dioxide. This process can also cause the pH to drop as the carbon dioxide combines with water to form carbonic acid. Excessive plant growth can cause the pH to drop to extremes at night, rising again out of the danger zone in the day as plants utilise the carbon dioxide in photosynthesis.

A drop in pH in a pond to below 7 will lead to dramatic changes in koi health, particularly if they are long term. Colours in koi are likely to fade through the deposition of excess mucus while in extreme cases, koi may even be seen to gasp at the surface. Acidic pond water is also likely to be corrosive to exposed plastic and metal surfaces, causing the pond water to become a toxic cocktail of contaminants.

What if the pH exceeds 9.0?

From experience, an excessively high pH is less extreme on our koi than a low pH, but should still be avoided. While a drop in pH is caused quite naturally, excessively alkaline water is likely to have been caused by a pollutant, with cement or builder’s lime the No.1 suspect. The symptoms of an excessively alkaline pond will cause fish to secrete excess mucus and also lead to gasping at the surface, very similar in fact to those when fish experience acidic conditions.


A buffer is a chemical that when added to a pond, will help to stabilise a suitable pH. The natural tendency is for a pond’s pH to become acidic. A buffer reacts with any excess hydrogen ions (if and when an excess arises) and will release them back again into the pond water should the pH rise too high. The most widely used buffer in and around ponds is calcium carbonate (CaCO3). This can be in the form of crushed shells, limestone gravel or similar material.

How a buffer works in your pond.

Let’s say, through extreme levels of respiration, carbon dioxide is released into the water. This combines with water, to form carbonic acid, which dissociates into 2 different ions.

Eg. H2O + CO2            H2CO3        H+ + H+ + CO32-

The free hydrogen ions have a potential to make the water acidic.

Adding a buffer – such as calcium carbonate (CaCO3) to ‘soak up’ those free hydrogen ions, prevents the pH from dropping.

By adding limestone gravel to your filter, the following will occur.

CaCO3 + H2CO3         Ca(HCO3)2          Ca++ + HCO3- + HCO3-

Limestone + carbonic acid           Calcium bicarbonate          Which dissociates into Calcium and bicarbonate ions (The hydrogen ions are no longer ‘free’ so do not contribute to the pond’s acidity).

This buffer will also work in reverse, releasing hydrogen ions back into the water should the water become excessively alkaline. For example, when the pH rises towards pH9, the calcium bicarbonate (which is now dissolved in the water) will release its hydrogen ions, causing the pH to drop slightly (but still remain alkaline), leading to a deposition of insoluble calcium carbonate onto submerged surfaces – including some plants.

Boxout: Diffusion

The molecules of a fluid (gas or liquid) are constantly moving. The molecules are so tiny that we cannot see them moving, but we can often see evidence of their movement, and thereby observe diffusion in action. For example, if we turned off all pumps and aeration in a pond, allowing the water to slow completely and added a dye in one corner of a pond, it would ‘move’ or diffuse from an area of high concentration to an area of low concentration through the whole pond until it became a uniform solution (reaching equilibrium). It is by diffusion that the appetising odour of food or the allure of pheromones will travel across large distances in a lake. But more importantly, diffusion plays a key role in the lives of fish and other aquatic organisms whose job it is to process pond water.

Diffusion is a ‘passive’ process which means that it does not require any input of energy from living organisms such as fish or filter bacteria and will occur whether life exists or not. The temperature of a fluid (ie pond water) determines at what speed the molecules move in their random directions with faster movement (and rates of diffusion) occurring in warmer water.

Boxout: Solvents, Solutes and Solutions

Water is an excellent solvent. Any substance that become dissolved in a solvent is called a solute. So salt (the solute) is dissolved in water (the solvent) to make a salt solution. A solution, if sampled at any number of points will be made up of precisely the same proportions of solute and solvent.

Boxout: The Chemistry of Biofiltration

NH3 + O2 à NO2 + 3H

This clearly shows that during nitrification in a biofilter, chemotropic (chemical loving) bacteria split the hydrogen ions from the nitrogen in ammonia and add oxygen (it’s an aerobic process) to form nitrite.

NO2 + O à NO3


Further bacterial action leads to the addition of another oxygen atom to nitrite to form less harmful nitrates.

Kill blanketweed and string algae.