My job affords me to see many beautiful bathrooms, unfortunately several of which have been ruined by careless maintenance. One comon problem is the unsightly ring stains formed by water stains and toilet tank that has accumulated dirt and rust. This dirt is carried by the water that flows from it and get deposited to your toilet bowls. These stains build up little by little especially in guest rooms that are not regularly used.
Since the mineral stains are alkaline by nature, mild cleaners are best solution to get rid of it. I recommend Lemon Eze by Ecolab. Check out this link on how to use it - Bathroom Cleaner
Note: do not use abrasives and use gentle scrubbing action. Use Lemon Eze to restore and to maintain your fixtures.