I work for a glass manufacturing wholesale company and what you have there, we call a "shell". If its on the outside of the tank, and not very deep, just fill it in with silicone to smooth it out to help with appearance. Test it outside, im betting you will be fine. That will be the weak spot on that pane, so if you feel like you have to do something, and you dont want to spend alot, buy a small strip of glass, say 4inches by the height of the tank and silicone it into that corner form the inside, covering the damaged area.