I would get at least 2 grades of rouge, maybe 3. Yes, the roughest one will scratch the glass although it may appear as just a haze or duller area. That is what you need to take out the scratch, or at least to dull the edges so it is less visible. The next 2 grades will be to restore the shine to the hazy area.
You really have to weigh the cost of the rouge and the elbow grease against replacing the tank when someone like Petco puts on their $1 a gallon sale. Or just turn the tank around. After all you will need to empty it to polish it anyway.