Try a pumice stone! They make special ones for this with a handle. I was skeptical when this was suggested to me, but it is the only thing that has worked for me. I found it in the store with the scrub brushes, mops, etc.
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I use Bon Ami for cleaning and like it, but it won't remove the ring. I tried Borax, vinegar and even resorted to buying a not environmentally friendly cleaner to try to get rid of my rings but even that didn't work. My toilets bowls won't fill beyond a certain level (they drain) and the ring is right at the water line, so using things that need to soak just didn't work. A mixture of borax and vinegar left overnight is supposed to, but since my water level won't stay beyond the ring line to soak, it wouldn't for me.
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Good luck!