Will you have a trim panel underneath (like a typical manufactured tank)? Or will it be 100% supported underneath without any trim panel?
The real answer to the question (as Mario is asking) depends on how the bottom will be supported because if it is 100% supported, I should be able to park my Bronco on it, but if it will only be supported on the edges, that is a substantially different (lesser obviously) weight because that 7' span is going to really allow the glass to bow. If the edges are support AND you have a support point at the one-third marks, the weight it can support will go up because the glass will only be supporting a 2.3' x 3.5' area and there will be less bow with the same weight as if there were no center supports.
As you can see, there is A LOT that goes into making a custom tank.
I'll give you $50 for a piece cut 16" x 20" with polished edges. :lol: