Added on: December 2nd, 2008

Are our Ponds becoming more Acidic?

Low pH
It has been noticed lately that pond keepers are having lower pH readings. What is the cause of this?

One theory is that now we have better control of Blanket Weed in our ponds, due to more efficient product’s to eradicate it.
The weed that was present in the pond would be continually dieing and regrowing, the dying weed would decompose and give off Ammonia, this would raise the pH.

Also new ponds tend to have high pH until the filters are mature (New Pond Syndrome).

In a mature pond the pH will fluctuate throughout the Day, the reasons are: Koi give off Carbon Dioxide from their gills which easily mixes with the water to form Carbonic Acid which lowers pH, a pond with Blanket Weed or other Plant life will take up the Carbon Dioxide and release Oxygen during the daytime, which will cause the pH to rise. During the night the plants will take up Oxygen and release Carbon Dioxide dropping the pH overnight.
That is why it is so important to have lots of Air in the pond in the Summer which will drive off the unwanted Carbon Dioxide, maintaining the pH.

The Ideal pH Range for Koi is 7.5 to 8.3

Carbonate Hardness
Another aspect of the pH in the pond, is buffering capacity of the water. This is controlled by KH (carbonate hardness).
This is an area where many people get confused. One of the reasons the term alkaline is avoided a bit and the term Basic is used for the pH scale is because this reading is measuring the alkalinity of the water. It is not the same as alkaline.

The alkalinity is a measurement of the waters buffering ability, or its ability to absorb and neutralize acid. Clearly the more alkalinity or the higher the Carbonate Hardness of the water the less likely you will incur pH swings in the water. It is therefore important to get this figure reasonably high to stabilize the water.

Ways to raise the pH (KH)

Aerate the water, driving off the carbon dioxide (CO2)
Filter over coral or limestone
Add rocks containing limestone to the filters
Use a commercial alkaline buffer

Many Koi keepers use a bag of crushed Oyster or Cockle Shells, suspended in a bag in one of the filter bays. This will need replenishing after a time, because of the effects of Carbonic Acid.
This is a massive and interesting subject, but it has been said before Koi Keeping is merely Keeping Water.

Happy Water Keeping.


  1. Colin December 3, 2008

    Excellent article highlighting the dangers of a low pH leading to a filter crash, as you say this seems to be happening more and more.
    Whilst you highlight the importance of the carbonate hardness to buffer the pH you do not mention that oxygen and the carbonate in the water are both essential ingredients in the nitrification process itself. This nitrifaction process also produces carbonic acid. In some ways it’s a bit like the chicken and egg question, is it the low pH that causes the filter to crash or is it the lack of carbonate present which stalls the nitrification of the ammonia.
    One other problem at this time of year is that usually quite a lot of ammonia is gassed off at the water surface but at this time of year, once the ponds are covered, these escaping gasses are trapped in the pond.
    Finally, on a recent pond visit several people were bubbling air through a cylinder containing LithAqua and claimed they were getting good results in buffering their water hardness.

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