Is the bottom of your toilet bowl dirty? It is likely that the problem is tartar. In fact, scale tends to settle very easily at the bottom of the bowl. Residues and dirt after going to the toilet cling to the surface and create brown stains at the bottom of the bowl.
If you do not take care to clean this part of the toilet regularly, it becomes difficult to remove stains. Fortunately, there is a solution to this! Whiten the bottom of the toilet with hydrochloric acid
Another quick way (and no more polluting) is:
see page 2
