You can use 10, 20, 30 or 40 volumes of hydrogen peroxide and it is a natural remedy that has surprising effects in cleaning the toilet and freeing it from 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 fumes. Put it in the toilet and leave it overnight , the next morning scrub with the brush  and then flush the chain, the bottom will be very clean.