I have a 115 gl tanks with all tempered glass. My solution was to remove the back glass, clean the tank from silicon traces and glue a new drilled glass.
As for the polarized glasses, I have a pair of polarized rayban's and couldn't tell the difference. So don't rest your hopes on a pair of glasses. I bought the tank today and I'm going to take it to get it drilled in a few days. I'll ask the guy to try to drill the back glass. I 've been told that the whole tank is made of car glass(tempered) and didn't see any difference. Its made in China so I'm thinking that maybe the Chinese wouldn't go and pay extra money for tempered glass. I will warn him that It could be tempered so he will be careful not to get hurt. Maybe he has a way to tell.
If the glass shutters then the polarized glasses definetely can't tell you if its tempered or not.
Hmm well I guess I could be wrong. This is from wikipedia.
Polarizing sunglasses reveal stress in car window (see text for explanation.)The photograph on the right was taken through polarizing sunglasses and through the rear window of a car. Light from the sky is reflected by the windshield of the other car at an angle, making it mostly horizontally polarized. The rear window is made of tempered glass. Stress in the glass, left from its heat treatment, causes it to alter the polarization of light passing through it, like a wave plate. Without this effect, the sunglasses would block the horizontally polarized light reflected from the other car's window. The stress in the rear window, however, changes some of the horizontally polarized light into vertically polarized light that can pass through the glasses. As a result, the regular pattern of the heat treatment becomes visible.
So in other words you can't tell by looking directly at the glass. you have to look through the glass at a different glass surface to see if its polarized or not.
Here is the setup to see if it is or not tempered.
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