Water that contains a high level of minerals is called hard water. Over time, it can leave unsightly spots and stains where the water sits or drips. Hard water stains can be tough and sometimes almost impossible to remove from your bathtub, but it is possible. This job, though, does require the use of a commercial cleaner.
Wash and rinse your bathtub very thoroughly to remove any debris besides the hard water stain.
Apply the diluted hydrochloric acid cleaner to the hard water stains according to package instructions. Use the sponge or a nylon-bristled brush to rub the cleaner on the stain. Rub in a circular motion to remove the stain.
Rinse the solution with a rag and running water. Dry with a towel. Repeat the process if any of the stain remains.