Next, pour some distilled vinegar into the toilet bowl. Soak a few paper towels in vinegar and place them around the rim of the toilet bowl, ensuring that they stick and stay situated there. Let the paper towels and vinegar sit overnight, then remove the paper towels in the morning. Next, scrub the toilet bowl with a toilet brush and flush. If you still have hard water stains, you may have to repeat the process. To see this method in action, check out the video below!
Credit: Home Remedies Easy