Plywood tanks are fine. Built one myself...still running fine. I read the same stuff you obviously have about using a pool, marine, or food safe epoxy. I digested all the info and then said to heck with it and just used a black two part epoxy I found at home depot. Primary purpose of my tank was as a proof of concept, so I rolled the dice...sounded like a neat idea. Learned a lot.
Whatever you choose, my advice is to put lots of coats on and make sure the epoxy is thick. I made the mistake of putting an urchin in mine....once the walls were all covered with coralline he started chowing down and actually scrapped through the epoxy in a few spots and exposed some wood. He was quickly moved, tank partially drained, spots covered with some silicone, everything's fine. It's a softy tank...xenia, shrooms, zoos, colt, ricordia, two clowns...actually there's some montipora in there too. All fine.
To all the naysayers....why are you down on plywood? There's plenty of horror stories of glass and acrylic tanks letting go, rios failing and wiping out a tank, etc....nothing's totally fail safe in this hobby....