Stains in the bathtub can occur from either trapped water not allowed to dry properly, which forms mildew, or from mineral deposits created by hard water. Regardless of how they got there, you don't have to live with dark stains in your bathtub indefinitely.
How do I get my bathtub white again?
Make a bathtub cleaner paste of one part hydrogen peroxide to two parts baking soda (or cream of tartar). Rub the paste on the stain and let stand for 30 minutes to one hour, then wipe and rinse.Feb 16, 2021