This is more home maintenance than repair, but maybe someone has a good solution.
After about 5 years of use, our new toilet bowls show rings around the inside water level in the bowl. There seem to be only two ways of dealing with this nuisance: a. mechanical abrasion with cleanser or pumice stone, or b. use of an acid that will dissolve the calcium deposits.
Both methods have a deleterious effect on the underlying porcelain, making the problem worse the next time around.
Is there a third method, which does not dissolve the porcelain in the cleaning process?