Pour into cistern, a full 16oz bottle of hydrogen peroxide. Let it sit in there for at least an hour or so. If that doesn't work, there is a product available online from Walmart (not sold in stores) called "Bolex - Toilet Bowl and Urinal Cleaner. Follow directions. Be sure to wear gloves and avoid inhaling the fumes and wear eye protection as the product does have hydrochloric Acid in the ingredients. This stuff is amazing at removing rust stains and scale build-up.
Thank you ! I appreciate you taking the time to answer & I will the first & see how it works for us. If not I try the second 👍