You have hard water. Minerals and such. We have it too.
Suggest using "Equate" dental cleanser from WalMart. 3 tablets in the bowl may just get rid of the stain. You can get plenty for under $3 easy.
It is the same thing as Polident false teeth tablets.
To prevent hard water, that is hundreds, since it involves softwater systems.
By the way, get the book Haley's Hints for this very same solution to your problem.
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