What size tank are you thinking?
How would you address the integrity of the top and bottom frame?
+1. I wouldn't try it. You're going to have to remove the plastic framing anyways, creating a frameless tank. You're probably better off to actually have the glass custom cut to the dimensions you want and then construct the tank yourself. The cost may be a little more up front, but in comparison to what it is going to cost you if the seal doesn't hold, with water damage and loss of stock, you'll be better off.
There is a chart somewhere that gives dimensions of tanks, primarilyy the depth, and the proper glass thickness to create a tank that will hold and not bow of that depth. I'll see if I can find it. I can't remember what site it was on. That was a long time ago that I saw that. I want to say it was MFK.
WYite