One of the recent woodworking magazines have an article about building a plywood and glass corner tank. Epoxy was used to coat the wood, just like on boats.
You don't really need marine plywood- a good exterior grade is fine.
Many people feel that if your inside paint / coating fails the wood part of the tank will need to be replaced either way, so you might as well save the money and use exterior grade plywood and use the savings to upgrade other aspects of the tank.