Its easier to tell if glass is tempered that if its not.
Definately tempered:
There is a sticker on the bottom that says, "Tempered Glass, do not drill"
There is an etching, usually in one corner.
Might be tempered:
The bottom pane is thinner than the sides.
Looking through phased polorized lens shows blue streaks.
Shouldn't be tempered, but ???
Glass is 1/2 inch think or more.
Glass is same thickness as sides. (Except for 55 gallon and smaller, some 55's use 1/4 inch tempered glass all around)
If you absolutely cannot tell and really want to drill the tank. First measure the bottom pane of glass and price, in your case a 3/8 inch piece of glass. If it is within a price you are willing to pay to have a drilled tank, drill away. If the glass shatters, just replace it. And drill the replacement. In the case of my first tank, an 80 gallon, I removed the glass because I knew it was tempered and replaced it. It cost me $60.
Mike
