Copyright 1999 AquaChem Publishing

robably the least understood water condition that occurs in an aquarium is that of water hardness. Most of the confusion about water hardness comes from the sheer magnitude of terms used to describe it, many of which are synonyms. Usually, the initial confusion arises from the fact that hardness can be measured either in parts per million (ppm) or in DH units. Each DH unit is equivalent to 17.8 ppm. DH, or Deutsche Hartgrad, loosely translated, means German hardness gradient, however most aquarists think of DH as Degrees of Hardness or German Degrees of Hardness (GDH). As a result of the different systems, it’s not uncommon to see water hardness incorrectly reported as DH = 85 ppm (an erroneous combination of both systems).

          Water "hardness" comes from metal ions that have a charge greater than one (i.e. Ca²⁺, Mg²⁺, or Fe³⁺ -- not Na¹⁺ or K¹⁺). The majority of water hardness comes from calcium carbonate (CaCO₃). This is mainly due to the composition of the earth’s minerals........................

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