1.5" will be more than enough. I ran the numbers on pure joint strength when designing a 12' long x 4' deep tank, and most people's fears are way out of line with the actual strength requirements of the tank.
I dunno about the maintenance on your typical reef system. I know in a FW tank I'd definitely hesitate to make it so tall just from a maintenance point of view - wanna get something off the bottom? gotta go in.
I have a 30" tall fw plywood tank and it is a real pain to get to the bottom. I need to use tongs because my arms aren't long enough to reach the bottom (unless I hold my breath and dunk my head in the tank - no thanks). At 70" tall you could drown in your tank doing maintenance!
Hey all, Update time, I have not given up but have to take a break from the build.
After the water test I found a leak that is under the panel. So I need to remove the panel and so far that has not been easy to do. I hope to work on it some more next year.
Sorry to hear than, man. Good luck on the fixing. I think with some external reinforcement, you'll be set.
I believe all that glassing might add strength to the seams but it doesn't help too much when it comes to the bowing. I think the bowing is what is breaking your window seal.
