My nephew was having fun feeding smelt and shrimp to my peacock bass... Next thing he said "uncle look" and he use my crystal decoration top sharp end and drew a 5" scratch on my tank in front of me.....
You can try some rubbing compound, like a car wax that are for deep scratches. If it is really deep use some really fine sand paper like 2000 or finer. Just soak the sand paper in warm water with a little bit of dish soap.