It is a pretty serious problem. The glass will eventually buckle and the seam will fail.
Too remidy this you can try a couple of different techniques. The easiest would be to buy a bar clamp and clamp it temporarily, then apply a two part epoxy to the cross brace. Give it about 48 hours to bond.
Other ideas would be to contact the manufacturer (
www.all-glass.com most likely) and have them send you a replacement top brace. You would likely need to drain out half of the water to do it, but it will be extremely secure when you finish.
I have tried using all sorts of other glues and bonding agents such as weld-on without success.
Your final option would be to replace the tank. As you said yourself, the fish are priceless. I wouldnt have too much of a problem spending $100 to gurantee they are in a solid tank. Maybe now is the time to upgrade to a larger tank also.
HTH