Calcium carbonate hardness
Water hardness is important to fish culture and is a commonly reported aspect of water quality. It is a measure of the quantity of divalent ions (for this discussion, salts with two positive charges) such as calcium, magnesium and/or iron in water. There are many different divalent salts; however, calcium and magnesium are the most common sources of water hardness.
Hardness is traditionally measured by chemical titration. The hardness of a water sample is reported in milligrams per liter (same as parts per million, ppm) as calcium carbonate (mg/l CaCO3).
Agricultural limestone can be used to increase calcium concentrations in areas with acid waters or soils. Agricultural gypsum or food grade calcium chloride could be used to raise calcium levels in soft, alkaline waters. Expense may be prohibitive when large volumes of water need treatment. At a pH of 8.3 or greater, calcium will come out of solution as an insoluble carbonate (limestone). Likewise, agricultural lime will be insoluble in waters with that pH range. Identifying a suitable water source may be more practical.