I have tried everything but I can't get the hard water stain out of

COKE

have you tried a descaler? It's sold at plumbing supply stores - not in your typical big box stores.

Any cola soda like coke or pepsi should help. You'll need to leave it sit a bit to do it's majic

Have you tried Ironout found in Lowes

Coca-Cola pretty much gets hard water stains out of anything (and your question was cut off abruptly so I don't know what you need to remove the stain from). Hope this helps ;)

I make a paste out of peroxide and Arm & Hammer soda. Let is sit on the stain overnight. Then rinse. If the stain is deep, do the process again.

Vineager and baking soda paste. Let it sit for 15 minutes. You'll be surprised and it's gentle not chemical

http://www.thriftyfun.com/Removing-Hard-Water-Spots-on-Windows.html