Water Hardness
Water hardness is commonly confused with alkalinity. The confusion relates to the term used to report both measures, milligrams per litre as calcium carbonate equivalent (mg/L CaCO3). Calcium carbonate is a general term that indicates the total quantity of divalent salts present and does not specifically identify whether calcium, magnesium and/or some other divalent salt is causing water hardness.
Calcium (Ca2+) and magnesium (Mg2+) ions are the most common factor that comprises hardness and aquarium test kits usually determine both ions as “total hardness”. Despite the much confused state of misinformation prevalent in the hobby, permanent or general hardness is true hardness; carbonate hardness (°dKh) is not hardness at all but alkalinity. Water hardness should be expressed as a concentration of divalent ions in mg/L (ppm). The terms “permanent”, “general”, and “carbonate” hardness should be discontinued.
Calcium and Magnesium are essential in the biological processes of fish (bone and scale formation, blood clotting, and other metabolic reactions). Fish can absorb calcium and magnesium directly from the water or from food. However, calcium is the most important environmental divalent salt in rainbowfish culture. The presence of calcium in aquarium water helps reduce the loss of other salts: for example, sodium and potassium from the fish’s blood. Sodium and potassium are the most important salts in fish blood and are critical for normal heart, nerve, and muscle function.
Research has shown that environmental calcium is also required to reabsorb these lost salts. In low calcium water, fish can lose substantial quantities of sodium and potassium into the water. A recommended range for free calcium in aquarium water is 25 to 100 mg/L (50 to 250 mg/L CaCO3 hardness). A low CaCO3 hardness value is a reliable indication that the calcium concentration is low. However, high hardness does not necessarily reflect a high calcium concentration. It is not clear however, whether calcium plays an equally important role in embryonic development.
Water hardening (swelling process of flaccid newly shed eggs when they first contact water and absorb water) of fertilised eggs varies according to the water hardness. When hardness is low, increase in egg diameter is greater. However, the effect of water hardness on larvae hatching and growth of rainbowfishes is unknown. For those species, which are found in hard water in their natural environment, hard water may be required for good development, while in others it might ameliorate the deleterious effect of non-optimal conditions.
Water hardness is expressed in a confusing array of scales, although in the aquarium hobby the influence is to express them in terms of milligrams per litre of calcium carbonate (mg/ L CaCO3), which is also equivalent to parts per million, or in the obsolete German units of “degrees of hardness” (°dH). °dH or Deutsche Hartgrad, loosely translated, means German hardness gradient. 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). For aquarium purposes, you can use the following conversion factors: °dH X 17.9 = ppm (mg/L) ppm x 0.056 = °dH