If the natural fluoride content of supplied water of a public water system is less than eight-tenths milligrams per liter of water, fluoride shall be added to such water to maintain fluoride content of not less than eight-tenths milligrams per liter of water nor more than one and three-tenths milligrams per liter of water beginning:
(A) On or before January 1, 1971, for a public water system supplying water to twenty thousand or more persons;
(B) On or before January 1, 1972, for a public water system supplying water to five thousand or more persons, but less than twenty thousand persons.