Cal-Mag is specifically designed nutrient to compensate for deficiencies Calcium and Magnesium (referred to as Cal Mag for plants) is almost always needed when using pure, distilled, or reverse osmosis water. Tap water almost always has an abundance of Calcium and Magnesium in it. Calcium and Magnesium are the most abundant minerals in tap water. The majority of the PPM, or EC reading, you obtain from a measurement device is Cal-Mag.