Stains can be such a challenge to remove and I put some to the test. Here are a few ways I found to remove these stains with inexpensive and easy to find ingredients.
Sometimes this is a bacteria and sometimes it’s a mineral. Pour some Hydrogen Peroxide (3% you can find at the store) into your toilet. Let it sit for a bit, scrub with a toilet brush and flush.
Spray with vinegar and with with a paper towel or scrub with a sponge. If you have tougher hard water marks, then I recommend making a baking soda and water paste. Place that on the hard water marks and let it set there for 10 minutes. After that scrub and rinse.