As far as I know, nothing can be done to repair it.
That being said, if you wanted to experiement...
I would try one of those kits they sell for repairing "bullseye" cracks in windshields. It contains a clear resin that might fill and hide the crack. Of course, the tank would have to emptied down to the crack, and be well dried.
I haven't tried this, but it seems like it might work.
I have a 105 gallon glass tank with a few scratches. They only really show up when algea starts growing in the scratch. I use the Kent Marine glass scrapper with the plastic blade and it keeps those scratches clean. So basically just keep the scratches clean and they will not become an eyesore.