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Odd that a tank that big didn't have tempered glass on the bottom. Was the bottom drilled or something? Did someone build that tank themselves?
A sheet of glass that big will not be cheap, but I'd replace it with tempered if you don't need it drilled. Tempered can handle more weight on it than the same sized annealed panel can.
You'll also want to make sure the stand it on is perfectly square and level before you put the tank back and if you're repairing the tank, that has to be perfectly square as well when done. Small stresses on one corner, edge, etc can lead to big issues later and you are seeing now.