Why does kH increase with temperature

Carbonate hardness (KH): Meaning, lower & increase

The Carbonate hardness, short KH, is one of the most important water values ​​in the aquarium, along with the water temperature, the total hardness and the pH value. The carbonate hardness gives that Acid binding capacity of the water. In addition, the KH is an important one Acid buffer. If it is set correctly, it ensures that the pH value remains stable.

In this guide you will find out why carbonate hardness is important, how to measure it, and how to lower or increase the KH.

What is the carbonate hardness anyway?

The carbonate hardness gives the Sum of all alkaline earth ions dissolved with carbonate or hydrogen carbonate in the water. The alkaline earth metals include calcium, magnesium, barium and strontium, where Calcium and magnesium makes up the relevant part of the alkaline earth ions in the aquarium. Other alkaline earth ions usually only occur in traces. The carbonate hardness is part of the total hardness and indicates the acid binding capacity of the water.

Importance of carbonate hardness

The carbonate hardness is one of the most important water values ​​in the aquarium. It serves as a buffer for the pH value and ensures that it remains stable. If you lower the carbonate hardness, the pH value also decreases. Conversely, if the carbonate hardness increases, the pH value rises.

Differences between GH and KH

When it comes to determining water hardness, one comes across the two terms total hardness (GH) and carbonate hardness (KH) in aquaristics. The total hardness gives the Sum of all alkaline earth ions in the water. The carbonate hardness is understood to mean negatively charged proportion of alkaline earth ionswhich is dissolved in the water together with carbonate and hydrogen carbonate ions.

If you are not interested in the chemical part, you should note that the total hardness stands for the total salt content of the water and therefore all of it influences organic processes. The carbonate hardness indicates only the hydrogen carbonate dissolved in the water and serves as a PH buffer.

Total hardness

  • Sum of all alkaline earth ions
  • Unit: ° dGH
  • influences all organic processes

Carbonate hardness

  • Sum of all alkaline earth ions bound with carbonate and hydrogen carbonate
  • Unit: ° dKH
  • “Controls” the pH value

Relationships: CO2, pH value and KH

The carbonate hardness is strongly related to the pH value and the CO2 content of the water. If you know two of the three values, you can determine the third using the following table, because the pH value results from the CO2 value and the KH.

In order to achieve a high pH value, the CO2 content must be reduced and the KH increased. For low pH values, the KH must be reduced and the CO2 content increased.

Measure carbonate hardness

The carbonate hardness should be measured once a week and after every water change. To get reliable values ​​you should have a KH water test in Teardrop shape (buy on Amazon *). Test strips often give inaccurate results and are unsuitable for an accurate measurement.

Lower the carbonate hardness

Many tropical ornamental fish species require soft and acidic water. Since our tap water has a pH value of approx. 7 and a medium hardness, the pH value and carbonate hardness must be lowered in order to create optimal housing conditions.


peat eliminates hardness builders in the water. By filtering through peat you can bring the carbonate hardness to very low values. Contains peat Humic acidswhich act as a buffer against changes in pH. This allows you to lower the KH to low values ​​without risking a fall in acidity.


Osmosis water is very soft and pure water. If you mix osmosis water with tap water, you can Adjust the KH and the pH value precisely and achieve a significantly lower pH value than with pure tap water. Alternatively, distilled water or rainwater can be used because they have similar properties to osmosis water.

Dennerle osmosis system

80.94 € (May 16, 2021 10:39 pm)

Soil soils

Soil soils lower the KH and thus also the pH value. However, after a certain time the capacity of the soil is exhausted. The KH can only be reduced slightly with soil.

Increase carbonate hardness

In some cases, the carbonate hardness has to be artificially increased. This can be the case, for example, if the KH is very low and you have one Prevent acid fall would like to. With an increase in KH, the acid binding capacity is reduced and the pH increased.

Water purifier

Through water purifiers like that JBL pH Plus, the carbonate hardness can be artificially increased.

JBL pH Plus

10.50 € (May 16, 2021 10:39 pm)