If it's from a table, I'd bet a heck of a lot that it's tempered glass. That being said, not all tempered has a label. If you're going to cut it yourself, and figure it's not tempered, I'd suggest researching how to tell the two apart. If you take it somewhere to have it cut, they should be able to tell you whether it is or not. 1/2" thick is safe for the 30" you want, but that's 30" from the top of the water to the bottom of the glass. If you want 36" of water, the glass would need to be from the surface down with the other 6" extending below the glass.