Ladybug,
Your 90g probably has 1/2 inch plate glass all around. I really don't know how much it can support. If you filled the tank, the water alone would weigh about 630 LBS. I'm gone to venture a guess here, but I would bet that the bottom can hold upwards of 1000 LBS. If the silicone sealant can hold the glass from blowing out behind all that weight, the glass has got to be incredibly strong.
I've made this offer to other people regarding driftwood. If you're willing to take the ride, you can pick through my driftwood and take anything you like. It washes up on the dock where I work and I get all kinds of interesting pieces. It lasts so long in the tank that I've built up a reserve that's going to take a decade to use up.
Bob, Philadelphia.