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). Calcium carbonate hardness 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. Hardness can be a mixture of divalent salts. In theory, it is possible to have water with high hardness that contains no calcium. Calcium is the most important divalent salt in fish culture water