hi is it on the inside of the tank.. you can go to a glass shop an ask for some polish powder an mix alil water. an some heavy elbow grease to remove some of it...
Once they are scratched they are scratched unless you could take it to a glass store like the fellow said in the second post but that would be a major pain in the ass unless it's a small tank. I'd just buy a new tank if possible and don't stack up any rocks in it so they don't fall and scratch your glass. Rocks + digging fish = scratched tanks.
to the OP, if you find a way please let me know, since i've been trying to work this one out for a while now due to some careless staff who scratched my tank!!!
It IS possible to buff out scratches on glass. Its just time consuming. I have done it with a right angle grinder and a buffing wheel using dry bar soap and toothpaste. It takes about an hour per sq ft. Laying inside the tank while doing this helps with fatigue and leverage for maintaining a constant and steady pressure.