Depending on origin and consistency of the subsoil, water may contain varying quantities of alkaline earth salts, usually in the form of calcium and magnesium salts. General hardness, by definition, is the sum of calcium and magnesium ions. Most fish and plants thrive well at a general hardness of between roughly 8 and 20°dH. Excessive total hardness values, should, however, be counteracted by the appropriate measures.
Recommended general hardness values:
Freshwater aquarium (community aquarium): 8-25 °dGH
Lake Malawi/Lake Tanganyika aquarium: 5-20 °dGH
Plant aquarium with few fish (aquascaping): 3-10 °dGH
Marine aquarium: not measurable with GH test because GH too high. Please test calcium and magnesium separately Pond: 6-20 °dGH