Hardness is commonly confused with alkalinity. The confusion relates to the term used to report both measures, milligrams per litre as calcium carbonate (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 and magnesium 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. 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. The major organs used are the gills for absorption and the intestine along with the kidneys. Calcium is the most important divalent salt in rainbowfish culture water and is maintained in the blood at levels higher than the environmental level. 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 also 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. However, it is not clear whether calcium plays an equally important role in embryonic development. A recommended range for free calcium in culture waters 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.
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, and degrees German Hardness (dH). For aquarium purposes, you can use the following conversion factors:
dH × 17.9 = ppm
ppm × 0.056 = dH
Total dissolved solids (TDS) is a measurement of the total amount of dissolved substances in water and expressed in ppm of NaCl. TDS is essentially the same as conductivity. For aquarium applications it is recommended that the conversion formula TDS (in ppm) × 0.64 = EC (in µS/cm) be used. The conversion formula is only an approximation. TDS meters are calibrated with a Sodium chloride solution.
Conductivity or specific conductance is the measure of the water's ability to conduct an electric current. Conductivity depends upon the number of ions or charged particles in the water. The specific conductance is measured by passing a current between two electrodes (one centimetre a part) that are place into a sample of water. The normal unit of measurement for conductivity is expressed in microsiemens per centimetre (µS/cm) or millisiemens per centimetre (mS/cm) . Conductivity meters are calibrated with a Potassium chloride solution.
© Copyright Adrian R. Tappin
Updated July, 2005.
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