The hardness of the water in the aquarium

The term "hardness" of water refers to the concentration, content, of calcium and magnesium in the form of ions in a given sample. with this term we refer to a total value, which can be divided into two others, the permanent hardness and the temporary hardness. The latter is missing after, for example, we boil our sample, in order to favor, through boiling, the elimination of the CO2 that makes up the calcium carbonate (in chemical terms Ca (HCO3) 2 + boiling = CaCO3 ( white solid) + CO2 (gas) + H2O (water)). during the boiling process the calcium carbonate is solubilized and remains, upon complete evaporation, on the edges of our container (limestone).
We are interested in the two hardnesses, and not their total sum, which take two names according to their characteristics: we will have GH which is equivalent to permanent hardness and KH which is equivalent to the measure of temporary hardness. The scale used, unlike the one commonly used by aqueduct technicians, is the German one. The reference that follows the result of our measurement will be:

for permanent hardness in German degrees = ° d GH

for temporary hardness in German degrees = ° d KH

these abbreviations are of fundamental importance to correctly refer to the two values, since if you speak to your aqueduct technician or to the technical office of your municipality to find out the quality of the water in your home, you must correct the value which is usually expressed in French degrees (° f).
This scale identifies various classes of water (total hardness)

The measurement of the KH value makes sense in fresh and marine water, while that of GH only makes sense in fresh water where the value is of fundamental importance for the correct growth of plants. The GH in sea water has values that are very, too high for our tests, and it is not practiced.
the correct values are different depending on the environment reproduced in fresh water, while the KH value should be around 8-10 ° dKH for the tropical marine aquarium.
The KH value for chemical correlations affects the concentration of carbon dioxide (CO2) dissolved in fresh water together with the pH. supplying CO2 to the system we can, at constant KH, reduce the pH value present in the aquarium, facilitating the breeding of some animals and the reconstruction of certain environments.