GH test

Description

A GH test measures general hardness. The US Geological Survey gives the simple definition: water hardness is the amount of dissolved calcium and magnesium in the water, and hard water is high in dissolved minerals, largely calcium and magnesium. FAO's site-selection guidance for aquaculture adds where those ions come from and how the reading relates to alkalinity: calcium and magnesium, which cause most of the hardness of waters, are generally associated with carbonate minerals which are the principal sources of alkalinity, and the equivalent CaCO3 values for the divalent Ca and Mg ions are often taken as a measure of total hardness. The part of total hardness chemically equivalent to total alkalinity is what FAO calls carbonate hardness, which is what a separate KH test reads. Where a reading falls depends on whose table is used, and the two authorities here do not agree. USGS gives 0 to 60 mg/L as calcium carbonate for soft, 61 to 120 mg/L for moderately hard, 121 to 180 mg/L for hard, and more than 180 mg/L for very hard. FAO's chapter, citing Sawyer and McCarty and Boyd, sets the same four bands at 0 to 75, 75 to 150, 150 to 300 and above 300 mg CaCO3 per litre. A tank at 100 mg/L is moderately hard on both scales; one at 140 mg/L is hard to USGS and moderately hard to FAO. The number a kit reports is the same either way, so the useful habit is to record the figure rather than the adjective.

Sources

• usgs.gov — https://www.usgs.gov/special-topics/water-science-school/science/hardness-water
• fao.org — https://www.fao.org/4/AC175E/AC175E09.htm

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