I'd guess you need to re-seal the glass.
The tank will need to be drained. Cut out the old silicone, clean the glass and contact area with rubbing alcohol, and apply a new bead of silicone. With the dimensions you listed and depending on the thickness of the glass, the pressure of the water may also be bowing the glass (causing the leak), if this is the case, attaching a support beam across the top and bottom on the outside of the glass to hold the glass in place will help. Even though it will slightly hinder the viewing area, you may want to add one in the middle too. Good luck.