Hard water occurs because of the dissolved minerals magnesium, calcium, and manganese in the water. Although this can cause hard water stains such as soap scum in the bathtubs and sinks, spots on shower doors, spots on mirrors, windows, etc this water is not harmful to humans. When you see hard water build-up on windows, you can normally remove them using some chemicals found in the home, or commercial products. For a large or complex problem, you could hire a window cleaning company for hard water removal of stains. These hard water stains can occur if the rain in an area has a large concentration of pollutants that have been dissolved or the sprinkler system hits windows routinely.