You can use 10, 20, 30 or 40 volumes of hydrogen peroxide and it is a natural remedy that has surprising effects to clean the toilet and get rid of the yellowish color that has accumulated at the bottom. The advantage of using this ingredient instead of products with chlorine or bleach is that it does not contaminate and does not produce toxic vapors. Put it in the toilet and  let it sit overnight  , the next morning  scrub with the brush  and then flush the toilet, the bottom will be very clean.