One part Dawn dishwashing liquid mixed with two parts HYDROGEN PEROXIDE. Add baking soda for scrubbing help. Use laundry brush to work into the stain. Supposed to actually take yellow armpit stains out of white shirts!
Hard water stains are very unsightly and often extremely difficult to clean. There are several household products that can effectively remove hard water stains in the toilet without the use of harsh chemicals.: Baking Soda and Vinegar