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This forum is the only one I go to for just about all my research. People here are great and everyone helps. The one thing that isn't the best is on plywood tanks. I would recommend going to a site that does much more on that end. Read up what people say and post for help there. The biggest thing you'll find is that most use epoxy and not pond liner. Try here: fingerlakesreef.com • Index page and good luck.
pond liner will work fine for a lagoon, and i've seen many lagoon style systems utilizing it, but how will you seal it where it meets the glass? the only issue i see is just that.
i would be afraid to trust that bond.
here is another link to a 581 gallon reef tank- DIY plywood aquarium, 581 Gallons
and another- http://www.garf.org/140.gallon.html