Stubborn Aquarium Reside

[Shelala]

I bought an aquarium second hand and I'm having issues with hard water stains being etched into the glass. I have had fish tanks all my life and I've never come across something I couldn't clean, until now! This is driving me bananas. I've tried vinegar, baking soda, Barkeepers abrasive powder and foam spray and I've even let it sit outside with fresh water in it for a week and it hasn't helped at all!!! Is there any advice you can give me?
Some people have tried fine auto sandpaper, 5000 grain or something. I have tried some fine sandpaper that we had kicking around and that didn't do anything either. A scraper doesn't work... Is it hooped???
It doesn't look horrible when it's filled, but it's a drag, because it is (or was) a beautiful 50g bowfront aquarium....

[elizae]

We have extremely hard water here, and so I have dealt with hard water deposits almost as thick as the glass on the back of my tank due to a leaky filter. The best way I have been able to take it off is to soak paper towels in white vinegar, leave them on there for 15-30 minutes, and to take a razorblade and do your best not to scratch the glass. Bowfronts are hard because they are not, well, flat. If you think about it, hard water comes from a high pH, so you need something with a low pH to dissolve it. It takes a lot of elbow grease to get it off, but it is possible.

If it doesnt come off, and you can barely see it filled, I think you should be just keep it your dirty little secret.