I would look carefully to see how much of the seal in the corner is detached. If it is a small area I would carefully cut off all of the filet of silicone from the inside of the corner, clean the glass as well as I could, even using a solvent like acetone to do a better job. Then I would apply a new filet of silicone along the whole corner.
The used 120 gallon tank I bought several years ago was "repaired" that way, and it never did leak in the 4-5 years I used it. The corners looked pretty bad, but I just made corner molding to cover them, from the same wood as my stand and canopy, and lightly cemented them over the corners with dabs of silicone.
Trying to remove and reglue such a heavy piece of glass on that big a tank would always be a distant last choice if I were doing it.