try using a long brace instead of a cross brace (or do both)
if the front pane is bowing, chances are the rear pane is too, so youll probably need to brace both the front and back of the tank with a strip of glass... the long brace's glue will have more contact with the glass and provide a better seal to stop it from bowing... the cross brace is more of a back up measure to provide stability.
ok so i thought i would just say that my 90 gal tank has been up and running for a while i used pvc pipe glue to hold it...it had a stronger bond when i used the silicone glue it just kept popping loose from the pressure