Cloudy Stain On Glass? Please Help!

[KevJ87]

Hello guys,
 
I recently was given a 29 gallon aquarium by a friend. The tank was covered with white and tan buildup, after 4 days of solid scraping and scrubbing and vinegar soaks I am proud to say that it is completed brand new again. 
 
There is only one problem left that is driving me insane and that seems impossible to fix.
 
On the back of the tank where he had a LFS background stuck on is an area where there is a cloudy stain on the glass. I thought glue at first but a razor doesnt really come into contact with ANYTHING when scraping it, and it is on the OUTSIDE of the glass.
 
I tried vinegar, lemon juice etc to no end and the result is the same, no progress.
 
I believe it might be some sort of mineral build up or something, it just appears as a cloudy mark almost like when glass is fogged up.
 
Can anyone help this situation is driving me crazy!

 

[attibones]

Here's how you fix it: put a black background over it. You'll never see it again.
Also, have you tried window cleaner? Sometimes we forget with our fish tanks that we can use these kinds of cleaners, so long as we don't sort the chemicals near the tank or wipe the inside of the tank with window cleaner and the like. If you spray it away from the tank onto a cloth, you should be able to get it. Otherwise, you can set the tank down on the front facing panel, soak a towel in vinegar and leave the towel pressed to the glass (you could place a plate over the towel to keep it pressing down), leave it for a few hours and it should have loosened.

 

[PrairieSunflower]

Nail polish
remover maybe?

 

[KevJ87]

Hi guys, thanks for the tips but none of them worked. It appears that once hard water ( lime / minerals ) have "etched" themselves into glass it is no longer possible to remove them. This should act as a warning not to let water spots sit very long! I contacted two different glass specialized shops in order to find out and both answered that people have brought in aquariums tops and what not before and they had to give them the bad news. 
 
Those of you who have hard water ( i don't but my friend does ) make sure you get those deposits off as soon as you see them!
 
P.S =   Here comes the black paint ! lol