Go to Lowes or Home Depot, etc and get some 1x2 OAK... maybe 2 pieces 24" long each. Don't get pine or another soft wood.
Use tape or a helper to temporarily hold each piece along the top edge of the tank, with the center point of each piece in front/behind the cracked brace.
Then, take a bar clamp like an Irwin Quick-Grip and close it down across the tank, pinching the pieces of wood against the tank... the pieces of wood will help spread the pressure of the clamp across the front and back. Clamp down with very moderate pressure.... just hard enough to hold the wood in place. Do not attempt to pull the bow out, you may shatter the glass with all the pressure on it. This should help prevent the bowing from getting worse until you get things sorted out.