If it is glass, I don't think you can buff it out at all. You can buff minor scratches out of acrylic, but if it is a deep cut, you can't get that out very well.
Did you scratch the side of the tank or the front/back? If it is a regular tank (not reef ready) and you caught the front/back, turn the scratched part to the back and put a dark paper background. When algea covers it, you won't even notice. If it's a reef ready, you really can't turn it around as it only looks right with the overflow in the rear.
hope you can figure something out,
Tangman