When hard water constantly sits in your toilet, it leaves behind dark black spots, which are the result of the calcium, lime and other minerals in the hard water. You may notice a black ring circling the toilet bowl, right at the water line.
Remove the water from the toilet bowl, by scooping the water from the toilet or turning off the water and flushing the toilet. Pour an acid-based toilet bowl cleaner directly on the stain. Let sit for several minutes and scrub with an abrasive sponge. Rinse with fresh water.
If you haven't cleaned your toilet for some time or have an old black water stain, you may need to use a pumice stone. Wet the stone in cold water and lightly rub against the stain. Only use a pumice stone on porcelain or china toilets.