I got it working! You enter your aquarium's dimensions in inches, and it tells you the recommended thickness for your side and bottom pieces of glass.
The bottom piece is calculated according to your tank being supported only by the four outer corners. If you're using a strong plywood support under your tank, you can probably use the same thickness of glass calculated for the sides, unless many holes are drilled.
It only does rectangular tanks, but it tells you the gallons and weight, too.
here's the link to the program:
http://www.esopenko.com/misc/SeanEso...Calculator.zip
It was created with Microsoft Visual Studio 2005 Standard (Academic) using the .Net framework 2.0. You can get that from here:
http://www.microsoft.com/downloads/d...displaylang=en
i accidentally uploaded an empty zip 
. Fixed that!