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Buffering Ponds



This article was compiled by David Hulse and published in Koi Carp Magazine, October 1999. I regard this as the most practical article for the average Koi keeper to understand the buffering of ponds and it is published here with the kind permission of Koi Carp magazine.

The buffering system of a pond is responsible for holding the pH value of the water at a constant value. Here we will look at how this system works and why it is so important for the health of your Koi.

Before the importance of the buffer system can be understood it is essential to explain the definition and chemical basis of pH. The pH is the degree of acidity in the water. The acidity is determined by the proportion of hydrogen ions (H+) to hydroxyl ions (OH-) in the water. Extremely acidic water (pH1.0) has very few hydroxyl ions, but many free hydrogen ions. Extremely alkaline water (pH 14) has very few free hydrogen ions but many hydroxyls. Neutral water (pH 7) has equal proportions of the two.

Thus free hydrogen ions lower the pH, increasing the acidity of the water. Free hydrogen ions are released by the filter system as a by-product of the nitrogen cycle. Carbon Dioxide, released by the fish, the filter bacteria and many other organisms in the water, dissolves making carbonic acid which lowers the pH. There are also many organic acids, such as humic acid in the detritus and sludge of a pond, which lowers the pH.

Plant and algal life in a pond can also have profound effects on the pH of the pond water. Although the plants and algae respire like animals, (uptaking oxygen and giving off carbon dioxide) they also photosynthesise. Here the plants use the energy of the sun’s light to fix the carbon molecule in carbon dioxide. The carbon molecule is then built up into a simple sugar, which forms one of the building blocks of the plant’s structure and metabolism. The carbon dioxide removal stage of this process requires sunlight for energy.


Thus, through the removal of carbon dioxide, there is less carbonic acid in the water so the pH rises throughout the daylight hours. At night time the plants cease photosynthesis, but continue to respire, thus there is a net gain of carbon dioxide. This dissolves in the water as carbonic acid and thus lovers the pH.

The pH of a pond is in a dynamic equilibrium. In other words there are many factors that exert an influence on the pH, and these are counteracted by the buffer system. The buffering capacity of a pond can be defined as the ability of the water to resist changes in pH. The buffering system is comprised primarily of bicarbonates and carbonates, giving it the alternative name of Carbonate Hardness. However other bases such as hydroxides, silicates, phosphates and borates contribute to the buffering capacity.

Another name for the buffer system is the Total Alkalinity. All the above mentioned compounds are strong alkalis (they are termed bases – they have a high pH). The total alkalinity is the sum of all these compounds.

How does the buffer prevent pH change?

Free hydrogen ions are effectively “mopped” up by the buffer, taking them out of solution and preventing them from causing a decline in pH (a rise in the acidity of the water). Conversely when the pH begins to rise too high, due to a lack of free hydrogen ions, the buffer system will “release” some to bring the pH back down to the correct level.

The more organic material that is in the pond, the greater the degree of acidification due to the release of hydrogen ions and organic acids, as well as carbon dioxide released as the waste matter decays. Thus ponds with a lot of sludge and detritus and ponds with high fish stocking densities exert a greater stress on the buffer system.

              Observe sensible stocking densities

How can I measure the strength of my buffer system?

There are two main types of test kits available. Those that involve a dipstick type test where a colour is compared to a colour scale, the more intense the colour, the stronger the buffer. The other type of KH test kits are those that work on a titration-type reaction. Here measured drops of an acid are added to a known sample of pond water. Mixed in with the acid is an indicator chemical, which changes colour from blue to yellow at a precise acidic pH. Thus the number of drops added to the sample needed to bring about the colour change is directly proportional to the strength of the buffer – or the ability of the water to resist a change of pH!

The buffer strength is usually measured in equivalents to calcium carbonate (CaCO3). This means that the pond water sample has a buffer that is a strong as a known concentration of CaCO3. The scale of this comparison depends on the manufacturer of the kit. Most choose the German scale of dH, where on degree equals 17.9mg/1CaCO3. Others chose hardness, where one degree equals 1mg/1CaCO3. The manufacturer should specify which scale is used, and what the given value means to your pond.

It is important to test the KH regularly, as the buffer strength is not a constant value. Constantly counteracting this downward trend will eventually use the buffer up. This will cause a pH crash that will ultimately kill your koi. Thus it is important to maintain the KH value of your pondwater.

What is the ideal level to maintain my KH at?

The ideal KH value for koi ponds depends on the hardness of the water but should be a bare minimum of 4 dH (equivalent to 71.6 mg/L CaCO3). Test the KH of your tap water. If the value is above 6 dH (107.4 mg/L CaCO3) then regular small water changes will ensure the buffer is constantly topped up and does not fall to a level where it can no longer maintain a steady pH. Also, certain Montmorillonite clays can top up the KH value if used regularly as directed by the manufacturer. However, if the KH rises too high then the pH will also rise which may cause health problems for the fish.

I live in a soft water area, how can I increase the KH of my pond?

Soft water has very low carbonate hardness, and thus cannot resist strong changes in pH, if your tap water KH is below 4 dH it will need a buffer replenisher. Buffer powders are now commercially available to koi keepers. These can be added to the pond to bring the KH value up to desired levels. However be careful not to add too much, as they may cause the pH to rise too high.

So Remember.....

The buffer system is essential to the long term health of the pond as it counteract the natural acidification of the pond water, which would ultimately counteracts the natural acidification of the pond water, which will ultimately kill the fish. The buffer system is an exhaustible resource, so regular replenishment by water changing or mineral additives is vital. Ponds with heavy fish loads or much accumulated organic matter will create a heavy load on the buffer system. Thus pond cleanliness and sensible stocking levels are a must.





Last Updated on Friday, 16 July 2010 23:15