The total hardness, GH (or TH in French degree but never used TH = 1.78 × 1 ° GH): it corresponds to the calcium and magnesium salts contained in the water and results from the accumulation of the carbonate hardness and the permanent hardness. The richer the water is in calcium and magnesium salts, the harder and therefore calcareous (commonly) it is.

A GH hardness of 5 ° to 10 ° corresponds to soft water. Some tests are displayed in German degrees, the tests giving this value. To convert TH into GH, multiply the TH value by 0.56 Carbonate hardness (KH in German degree or TAC in French degree): these are the carbonates and bicarbonates of calcium and magnesium which disappear after boiling water . They are therefore temporary. KH acts as a buffer on the pH. Permanent hardness: these are the calcium sulphates and chlorides which remain present after boiling. The pH: it corresponds to the degree of acidity of the water. Below 7 the pH is acidic, above 7 it is basic, equal to 7, it is neutral. The pH varies during the day: at night it decreases because the plants reject CO2, during the day it rises because the plants consume the CO2 for photosynthesis. The most important parameter to take into account for the welfare of the fish is the total hardness. It is necessary to respect the requirements of the fish with regard to this parameter. The pH must also be respected, especially for very delicate species such as the Discus for example. Axolotls need hard water that is either hard (GH = 10 to 15) and basic (pH> 7). Below 10 for the GH we are in soft water, that is to say going towards acidity. However, the minerality of water is essential for the proper functioning of the axolotl organism and especially of its digestive system. Above 20 we are on a very dangerous GH on the other hand, be vigilant. Recurrent yeast infection is often caused by inappropriate or abruptly changed GH.