Are the scratches on the inside or the outside? You could try various rubbing compounds, while I was a glassblower we had some successes with toothpaste and a polishing head on a dremmel tool, this how ever often just make things worse by making a dull spot. If the scratches are on the outside or the tank is empty you could try Hydrofluoric acid, if you can get your hands on it. This will actually eat away some of the glass, you would have to be careful of your hands, eyes ect and if you use to much or do not take it off quick enough it could distort the glass enough that you would be able to see it and perhaps even weaken the glass. We used it for special calibrations and for cleaning glass; you can also dilute it with water. If you can get some and would like some instructions let me know.
