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Alkalinity         (Alkalinity)


pH and KH
KH JBL test kit 250pH and KH levels are probably best considered as two parts of the same parameter.  Koi can adapt to any pH between 7.0 and 8.5, but if the pH changes too quickly, they will become stressed. The pH should be kept within this range and also not be allowed to vary by more than 0.2 per day.  If greater variations than this continue over an extended period, the fish’s health and development will suffer.  If the KH is kept above 90 mg/L (5 KH) the pH will tend to be stable.   The results of KH tests are usually given in degrees of KH (dKH) and these values can be converted to mg/L by multiplying by 17.85 (multiplying by 17.9 is close enough).

Maintaining KH levels at 90 mg/L or above will avoid the risk of pH variations or a pH crash.  Low values of KH can be maintained as long as close attention is paid to the pH in order to ensure that it doesn’t vary and isn’t in immediate danger of a pH crash.

Soft water which has a low GH and KH and a consequential low pH near to 7.0 is often considered to be more in keeping with Japanese mud ponds so soft water is certainly not detrimental to koi health. In fact, many breeders would say that low values are better. However a pH crash could prove fatal so hobbyists who prefer lower values of pH and KH should not necessarily adjust them to higher values but do need to be aware that their pH and KH values should be checked regularly.

Hardness shock
If koi are transferred from soft water into hard water or if the KH is raised too quickly, the shock of the sudden change in KH will cause them to become stressed and they will sometimes sit on the bottom and possibly roll onto their sides.  The effect isn’t usually life threatening.  Koi will adjust to the change and recovery takes about three days.  Hardness shock does not occur in the case of a koi being moved from hard water into soft water.

If the KH value of your pond needs to be increased, this can be done easily and safely by adding sodium bicarbonate at a rate of 30 gm per 1,000 litres per day until the required value is reached.

To raise the KH in a pond without the risk of stressing the fish requires a little mathematics.
Target KH (mg/L) – Current KH (mg/L) x Volume (litres) = Amount of carbonate required.


If the target value is 100 mg/L and the current value is 20 mg/L then, for every 1,000 litres of pond volume:

100 mg/L (target value) - 20 mg/L (current value) x 1,000 litres = 80,000 mg (or 80 gram)

Note.  The maximum rate of KH increase should be limited to 20 mg/L per day in order to avoid the stress of hardness shock.

The example above will give an increase of 80 mg/L (100 - 20 mg/L).  In this example, unless there is a problem that poses an immediate threat to the lives of the koi such as a pH crash, this amount should not be added all at once.  It should be divided into four equal parts and added over four days so that the KH increase does not exceed 20 mg/L per day.

A convenient and safe way to add this amount of carbonate would be to use sodium bicarbonate which is sold in supermarkets and grocery stores.  Sodium bicarbonate has the chemical formula NaHCO3 and it contains only marginally more than 70% carbonate (CO3), the rest is sodium (Na) and hydrogen (H). The actual percentage is 71.43% if you wish to be picky, see the panel below for the chemistry explanation.

As sodium bicarbonate only contains approximately 70% carbonate, if we were to use it to increase the KH, we would need to multiply the amount of carbonate by  a correction factor of 1.4 (100 divided by 71.43%) in order to calculate the amount of sodium bicarbonate that will have the same effect as the amount of carbonate that has been calculated.  For easy to remember amounts the correction factor of 1.5 times is close enough and will give a perfectly acceptable rise in KH for koi.

In the example above, multiply the required amount of carbonate by 1.5 to get the amount of sodium bicarbonate that would raise the KH by an equivalent amount.  I.e. 80 grams x 1.5 = 120 grams.

So to raise the KH from 20 mg/L to 100 mg/L would require 80 grams of carbonate or 120 grams of sodium bicarbonate.

In a biofilter, the two nitrifying bugs (nitrosomonas and nitrobacter) convert ammonia to nitrite and then to nitrate. Between them, as they do this, they consume about 7.2 mg of carbonates for every 1 mg of ammonia converted. Since 100 grams of a typical koi food, containing 38% protein, when eaten and metabolised will produce about 4 grams of ammonia, this means that, for every 100 grams of food fed to your fish, nearly 29 grams of carbonates will be consumed.

Molecular weight of sodium bicarbonate (NaHCO3)


Molar mass g/mol

Number of atoms per molecule

Subtotal mass g/mol

Subtotal mass %





















From the table above it can be seen from the subtotal mass percentage column that the combined total percentages of the sodium (Na) plus the hydrogen (H) component is 28.57% (27.37% + 1.20%) of the sodium bicarbonate molecule.

The carbonate (CO3), is the part of the sodium bicarbonate that raises KH, and this is made up of one carbon (C) atom plus three oxygen (O) atoms and the combined percentage of these is 71.43% (14.30 + 57.13%).

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