Water hardness is made up of calcium and magnesium dissolved in water and is the key cause of most water problems in hydroponic facilities. High levels of hardness in untreated water lock out key nutrients to plants and also form scale on equipment and tubing. Calcium and magnesium are often the most abundant hardness minerals in tap water. The most typical form of these minerals in water is calcium carbonate and magnesium carbonate, and is often measured in grains per gallon (gpg). The molecules of these compounds found in untreated water are far too large and immobile to be absorbed efficiently by the root systems and transported to where the plant needs them.
For hydroponics, hardness is best removed by a reverse osmosis system which will reduce hardness by over 95%. While a softener will also remove hardness, it will substitute sodium for the hardness, making it difficult to grow plants with water that contains high levels of sodium.