Stains can develop on bathtubs and commodes if your water has a high level of sulfur. This happens when the sulfur in the water corrodes the plumbing and pulls iron from it. The iron is what makes black stains in tubs and commodes, but it is caused by sulfur water. Sulfur water is generally accompanied by a strong rotten smell. Use acidic cleaning solutions to remove these alkaline stains, but don't use any cleaning solution that will damage porcelain, fiberglass or plastic tubs or commodes. Test the cleaning method and solution first to make sure it won't cause any damage.
White vinegar is a mild acid that will remove black iron stains caused by sulfur water. To use it to clean a toilet, add three cups white vinegar to the toilet and scrub it completely with a toilet brush. Let the white vinegar solution sit for several hours and flush the solution down the drain. Repeat the process until the iron stains are gone. To use it in tubs, mix 1 part white vinegar with 1 part water and scrub it over the tub. Rinse the tub after the solution dries and repeat the process until the iron stains are gone.
Lemon juice is another acid that can remove iron stains from sulfur water. It is food grade, completely nontoxic and it will give your bathroom a pleasant odor. To use it, add one part lemon juice to one part water and spread it over your tub. Scrub it until the iron stains are gone. Let it sit if the iron stains are particularly pervasive. To use it to clean commodes, add three cups lemon juice to the toilet and scrub it over the iron stains. Let it sit until the iron stains are gone and flush the toilet.
Commercial hard water cleaners contain harsher acids that will remove very tough iron stains in commodes and tubs, but always check with the manufacturer to make sure it won't damage your porcelain, fiberglass or plastic surfaces. To use it, mix it with water and spread it over the surface of the commode or tub until the iron stains are gone. Rinse the area with clear water.
Borax is a powdered cleaning solution that will help grind away the iron stains. To use it, mix a paste of borax and lemon juice or white vinegar. Rub the paste into the stains on the commode or tub until they are gone. The borax forms a scouring powder that will grind away the marks. Rinse the area once the stains are gone.