Yeah, FR got us pretty good on that fresh tank frame then putting a really flimsy tank in there. Mine was the same way, the tank actually was drooping through the middle of the frame.
My frame collapsed during filling, my fault it turns out, so I rebuilt it. I added extra cross braces, dropped the corner where the drain/pump inlet is so that more water could get out and finally I added a 1/4" sheet of plywood across the bottom of the frame to give the open areas some support. So far, so good. Those legs of square tubing going left to right? I added those.