The 3 inches on either side of a 72 " long tank contain ~45 lbs of water/sand. The only thing keeping that weight off the floor directly beneath it is the glass bottom. Add in the actual weight of the glass itself and you are close to 60 lbs. That might indeed be enough to cause a fracture along the bottom. It's more likely to cause seals to give first.
My guess is that acrylic would handle that much better than glass, but only because of how its cast and it's ability to handle more deflection.
Engineers could calculate this, but really, it's just a smart move to not test structural limits with a half ton of water in a bedroom!