I prefur euro braced tanks so I tend to go along with the suggestion to replace the brace entirly with a piece of plexiglass attached to the walls of the tank itself but the intent of the brace is to prevent the glass from bowing OUT. Other than for purely cosmetic reasons theres no reason why you cant cut a thin strip of plexiglass and run it along the top edge of the broken brace and then glue it in place with a 2 part epoxy. I wouldn't bother with screws or rivets they will only provide a new place for cracks to start up. Once the epoxy is set up theres no danger to the fish. in any case the repair will be well about the waterline.
