GH Total Hardness Test. Hardness is primarily the concentration of calcium (Ca++) and Magnesium (Mg++), the two most important metal cations with more than a single positive charge.If the amount of these salts is low, then the water is called “soft”. When their amount is higher, the water is called “hard”. Total hardness is normally given in German degrees (dH), whereby 1° dH is equal to 10 mg per litre dissolved calcium oxide. The concentration of these salts in water effects the biological functions of plants and fishes. The optimal value for the total hardness depend on the species of fish and plants. Where many African and South Asian species flourish in relatively hard water, south American ones may need soft water. If a range of hardness of 3° dH to 16° dH is accepted, a range of of 4° to 10° dH is considered to be optimal. The Elos AquaTest GH makes it possible to quickly determine the buffering capacity of your aquarium water. • Precise, thanks to the calibrated droplet • Simple to use • Equipped with precision measuring vial and syringe • Practical thanks to the easy-opening pack.