With plywood you might want to use epoxy or fiberglass resin. You can even use the fiberglass to create a nice background.
I used the two part epoxy that I mentioned above. Click on the link I posted and you will see a description and pictures of some of my stages of production of the tank. I ended up having to use the epoxy paint to seal up the bubbles I was getting in the fiberglass resin. My tank has been running now for almost 2 years with frogs in it . . . no problems. You just have to make sure that the stuff cures well before using it (or at least that has been my experience).DaFrogMan said:Homer said I could get some epoxy from lowes for about $15-20. I'm affraid that epoxy will have chemicals that will hurt my darts.... anyone used it? If so, which brand is safe? Or are they all safe? Do I have to watch out for certain brands of epoxy or ingrediants in epoxy because they are toxic or anything?