I have a hard time seeing how plywood will be cheaper than glass for a frag tank. 1/4" glass is awfully cheap and a gallon of that epoxy paint is pretty expensive. Glass is also a lot easier and you'll have less exposure to the fumes which will shorten your life
Typically you can get 1/4" glass cut to size for no more than $5 per square foot. I suggest pricing this out before going though the trouble involved with a plywood build.