Your build looks great, and I'm happy to see you used fiberglass reinforcements - it probably wouldn't have held water at all without it. Loving the height of it - but I wouldn't wanna be the guy who had to clean your gravel!
Something I'd encourage you to read is this: http://www.monsterfishkeepers.com/f...11-Epoxy-Sealant-on-Plywood-a-cautionary-tale <- basically shows what straight epoxy sealant on plywood looks like after a few years in service. You might not have the same issue but I'd suggest draining most of the water and climbing in to have a real close look once a year or so to make sure this isn't happening to you.
thanks for the link, yea ive thought about painting it with liquid rubber for added protection, i might end up doing it either now or if i see anything wrong with it in the future.