The carbonate hardness (not the general hardness!) contains components which stabilize the pH level. They prevent extreme pH levels and ensure the aquarium functions properly.
What is the benefit of the correctly adjusted carbonate hardness?
A KH between 5 and 12 °dKH gives the water a stable pH level, so that the pH level hardly fluctuates. This stable pH value in turn enables the filter bacteria to work optimally, the water values improve and the fish are less susceptible to diseases. Stable pH levels prevent a sudden drop in acidity (drastic drop of the pH into the acidic range) and an extreme rise into high ranges around 10.
How can you increase the carbonate hardness?
It’s very easy to increase the KH, if it is under 5 °dKH: Through the addition of JBL AquaDur or JBL Aquadur Malawi-Tanganjika (a mixture of various mineral salts) you can continuously raise the KH of your water to values over 5 °dKH. Just add JBL Aquadur into the aquarium water, where it will dissolve. In a final measurement you then will find a value between 5 and 12 °dKH.