i have an old tank laying around but its got an awful hard water stain on it. normally i can get rid of them by either a scraper, vinegar or both but this one wont go away. its very thin, i can see through it, but no amount of vinegar will do anything and nor will a scraper. i remember a thread was posted a while back similar to this, and someone recommended a product, but i can't find it. anyone? thanks!