A water company, as defined in section 25-32a, shall add a measured amount of fluoride to the water supply of any water system that it owns and operates and that serves twenty thousand or more persons so as to maintain an average monthly fluoride content that is not more or less than 0.15 of a milligram per liter different than the United States Department of Health and Human Services’ most recent recommendation for optimal fluoride levels in drinking water to prevent tooth decay.