we use Jewlers Rouge mixed as a thick paste with water
There are scratch removal kits availabe in the trade but they cost around the $1000 mark.
What they do is grind the glass out makeing a small dish in the glass (in other words, makeing the glass thinner).
This can cause the glass to give an altered view of the inside of the tank, Just as glasses do or when you look at the corner of a curved corner tank.
As a general guide, if you can feel the scratch with your finger nails, it may be too deep to remove the scratch without damaging the surface.
The important thing is to use a pure wool buff on a slow speed (and I mean really slow speed 150 t0 300 RPM) drill. The use of a higher speed drill can cause the glass to burn and therefore you can get a rainbow effect in the glass.
You may well lighten the scratches without removeing the water from the tank but if you are going to have a serious attempt, remove all the water to avoid termal breakage