M.
K.,
First pour some bleach in there and let it sit for a while. Then you need a pumous stone. You can find one in the cleaning aisle usually but if not it is in the nail care aisle, one that you would use on your feet will work fine. Scrub on the stains with it and they will come off. It might take a bit of work but it definately does the trick! I had hard water stains on my toilets and was able to get them off after a couple sessions. Hope this helps.
M.