WATER SOFTENERS are used to treat hard water . They work by replacing the calcium and magnesium in the When water describes as "hard" is high in dissolved minerals , specifically calcium and magnesium . Hard water is not a health risk , but a nuisance because of mineral buildup on fixtures and poor soap and / or detergent performance . As water moves through soil and rock , it dissolves very small amounts of minerals and holds them in solution . Calcium and magnesium dissolved in water are the two most common minerals that make water "hard". The degree of hardness becomes greater as the calcium and magnesim content increases