Ok... I think I found the answer. This got a little confusing but I think it works:
When selecting a glass window the most important factor is water pressure.
It doesn't matter how wide or long the tank is at a certain depth it will always have a certain pressure no matter what as long as your using the same liquid with the same circumstances (i.e. temperature). This basically means that whether we fill a one inch pipe with water that's thirty feet tall or we swim thirty feet down in a lake the pressure will be the same.
Your tank will hold roughly 120 gallons of water and if we times that by the rough weight of a gallon of water we get about 1000 pounds of water. The amount of weight the bottom of your tank will have to hold is roughly 1.74 psi.
This formula gives the safe load of a rectangular piece of glass that is evenly supported on all sides.
P=84000t^2/Af
P is uniformly distributed load in psi
t is thickness of the glass in inches
A is the area of the glass in square inches
f is the factor of safety, usually its set at 10
Your equation is as follows if you were to use a 1 inch thick piece of tempered glass:
P=84000(1)^2/(1152)(10)
Which would make the glass able to support 14.58 psi which is way way over the 1.74 psi that the bottom of your tank would have to support. That equation basically means that if you had a 1 inch thick piece of glass how much psi could it hold with a safety factor of ten.
From what I've read most aquariums are built with a safety factor of 3.8 but I'll leave it at 10 for now and finish the equation to see what you should get. After completing the equation it said you can use a glass piece with a safety factor of ten that's only 1/4 of an inch thick. That's assuming though that nothing will ever touch or especially hit the outside of the glass once its in place.
I'm too lazy to finish the equations for if someone were to strike the outside of the glass but with those in mind 1/4 inch is not even close to safe for use in a fish tank. If I've done the numbers right if you were to go with 3/4" or higher there should be no problems.
