One thing I'd like to point out... I'm not sure how the tank will fit preciesly, but you'll want to be able to crawl under the tank and stand up behind it if possible (I.E. Don't fill the room right to the back wall.) If it eaver leaks, or you need to get at the plumbing or wiring, or whatever, being able to access the back side without cutting a hole through the wall is a major plus (speaking from experience - wives don't like holes in walls to get at fishtank plumbing.).
Other than that it looks really good. I doubt you'll need any extra support in the way of joists. I'd suggest the 3/4" marine grade ply instead of 2 layers of 1/2. Just make sure the seam falls on a joist, maybe do a double joist on the seam to give you extra fastening room. Make sure your marine grade ply is DRY before you fiberglass it, and you should be good to go! The marine ply is more expensive, but it's A grade ply with no voids, and should be chemical treated to be rot resistant for a loooooong service life under your tank where it may be damp without you knowing it.