°KH – Carbonate Hardness

KH – Buffering capacity, temporary or carbonate hardness in the water.

Also known as ‘total alkalinity’ or ‘acid-neutralizing capacity’ (ANC) in some countries.

The ‘K’ in KH comes from the German word ‘karbonate’. KH is a measure of bicarbonate (HCO3-) and carbonate (CO32-) ions that act as buffers in the water to prevent the pH dropping or changing sharply (especially at night if you have plants in the aquarium). One degree KH is equal to 17.9 mg/I (ppm) CaCO3. It’s also measured in degrees. The degree symbol may be replaced with a d (ie. 2 dKH).

A common misconception is that KH is a part of GH and that KH cannot be higher than GH. There is no such correlation. In some areas, the water contains more sodium bicarbonate and/or potassium bicarbonate than total calcium and magnesium. In these areas, the KH is naturally higher than the GH. Furthermore, people using water softeners will most likely have a KH that is higher than the GH, as water softeners exchange sodium or potassium ions for calcium, magnesium, and other hard water minerals.

Low KH

In tanks with a low KH value, say under 2-3d (35.7-53.6ppm), pH crash is more likely.
KH is consumed by nitrifying bacteria 24/7 so either your substrate generates more, you perform water changes to supplement it or you add chemicals to add KH to your aquarium as otherwise it causes a pH crash/shock.
Tip: get up just before the tank lights (or sun light) come on and measure the pH and observe the animals for stress. It may be substantially different than it is during the middle of the day.

Raising KH

If you live with soft water then you may need to raise it before you add aquatic animals to it. Measure the KH value and if it’s less than 4d (71.4ppm) then add some of the items below to increase it.

Remember that KH always tends to go down over time (See old tank syndrome).
Sodium bicarbonate (NaHCO3) (baking soda/bicarbonate of soda) is the usual KH additive of choice. It’s cheap and easy to find and use. It will however raise pH though due to the addition of the hydrogen.
Potassium bicarbonate (KHCO3) is another KH additive. It’s cheap. But not so easy to find, it’s a specialist food additive for people who don’t want sodium in their food. It will however raise pH though due to the addition of the hydrogen. It dissolves much faster than the Potassium or Calcium carbonate powders.
Potassium carbonate (K2CO3) A known food additive. This adds carbonate without raising pH. The potassium is a bonus if you have a planted aquarium.
Calcium carbonate (CaCO3) This adds carbonate and calcium so GH and KH will increase. Used by gardeners as ‘Lime’.
Reducing KH
The ions that make up KH can be removed by boiling the water. Boiling may also reduce GH slightly.

KH is also reduced by the action of nitrifying bacteria and by water surface agitation.

KH Calculator
Buffering capacity and pH
Notes
This water parameter is often ignored by many aquarists. But too low a KH can cause pH shock disease and death in aquatic animals.