You really need some cerium oxide - Brasso etc is unlikely to touch it. When I used it on mine it took a fair while to get the scratches out working with a power drill. Safest is with the tank on its side on a carpeted floor, that way you can apply more pressure without fear of breaking the tank. You do need to watch the heat build up though & keep it nice & wet while you do it.
If you are in the UK then
www.screwfix.com sell a glass polishing kit for about £20
I.