Is the brace glued to the top of the tank or inside? Did you use silicone or something else?
i used silicone and put a bead along the top edge of the tank. put the bracing in place on the long ends first, let sit overnight, then did the short sides, let sit overnight again. for the short sides, i braced a small piece of 2.x4 under it so it would have something to sit on while it was curing. cleaned up all excess silicon with sharp razor next day. its very sturdy and the tank did not bow at all when filled up. i'm thinking of doing one with a 20 long next!