I wouldn't worry about a centre brace. Acrylic tanks will always have some flex to them. To brace it you would have to empty the tank, cut a piece of Acrylic to width and then buy a bonding agent that will form a plastic weld. Talk to a store that specializes in plastics, fiberglass, etc. In my area there is a franchise called Industrial Paint & Plastics. I guess a temporary fix would be to build a wood brace to go across the top and overlap the edges of the tank. Make it the width of the tank and then squeeze the tank where it bows just enough to slide the brace on. If that proves too difficult then lower the water level until the tank is back to its original shape, add the brace, and then refill.