.5" tempered glass basketball backboards sounds like the perfect material to make an aquarium out of, they are designed to hold a load and take a beating without breaking... the same cannot be said about plywood, plywood can soak up water, rot and also would be difficult to attach the glass to as silicone probably wouldnt afix to it very securely
if you got ahold of enough tempered glass to make the four walls and the bottom of the aquarium, you could probably plug the holes fairly easily with smaller panes of glass/acrylic put over the holes on the inside of the aquarium with silicone to hold them in place, the pressure of the water would keep them from leaking so long as the patches were thick enough to hold back whatever amount of water you have in the aquarium
I think that it could be an awesome project, just dont underestimate the weight of a few hundred gallons of water (its a lot)