you can always look and see what they sell at auto window repair and replace shops. My friends old man works at one, and the sell stuff that can fill cracks, rock chips etc, with no damage, like it never happened!
A great idea for glass tanks
but acrylic is a different story-- and much more likely to scratch
the best way I have heard is to get some SUPERFINE sandpaper and sand the scratches.. and then buff them out