I'm pretty sure the OP is talking about building an aquarium out of plywood.
I feel that the cheap stuff isn't a good idea, depending on the size of the tank the plywood thickness would come into play. I wouldn't exactly cheap out on the type of plywood. I would think structural or marine plywood in higher grades would be a good starting block.
I don't have experience putting one together, but I've heard about the idea, just haven't done the research into it.
I think an important aspect would be the sealing of the plywood as well.