Since it is important to know both the GH and KH, the use of total hardness can be misleading and should be avoided. Other ions can contribute to GH but their effects are usually insignificant and the other ions are difficult to measure. In the U.S., hardness is usually measured in ppm of CaCO3. A German dH is 17.8 ppm CaCO3.
General hardness is the more important of the two in biological processes.
Incorrect GH will affect the transfer of nutrients and waste products through cell membranes and can affect egg fertility, proper functioning of internal organs such as kidneys and growth. Within reason, most fish and plants can successfully adapt to local GH conditions, although breeding may be impaired. In freshwater aquariums of neutral pH, bicarbonate ions predominate and in saltwater aquariums, carbonate ions begin to play a role. Thus, in practical freshwater usage, the terms carboante hardness, acid binding, acid buffering capacity and alkalinity are used interchangeably. The hydrogen ions freed in the reaction then increase the concentration of hydrogen ions and reduce the pH. Since nitric acid is the end product of the nitrogen cycle, this explains why aquarium pH tends to decrease and nitrates tend to increase over time.