Aquarium Carbonate hardness

The carbonate hardness (KH) is the amount of carbonate and bicarbonate ions. For these anions to act on carbonate hardness there must be a corresponding number of calcium and magnesium cations. The carbonate in the water is an acid buffer, which prevents a sudden and rapid change in pH. Water with a low KH value generally has a low pH value, while water with a high KH value is usually alkaline.

The total hardness (GH) is the sum of all the dissolved ions of the alkaline earth metals in the water. These include in addition to magnesium and calcium (about 80%), strontium and barium. The total hardness in an aquarium usually plays a lesser role than the carbonate. Normally the carbonate hardness is less than the total hardness.