General water hardness is the measure of calcium and magnesium ion concentrations in water. These ions are present in all waters except for deionized / distilled water. A reading of greater than 200 parts per million (ppm) will have a high concentration of calcium and magnesium ions. This water is hard water. Where as a concentration less than of 100 parts per million (ppm) is soft.
One ounce in 7,350 gallons of water……..
Mol Biol Evol. 2016 Jun 13. pii: msw110. [Epub ahead of print]