You could try to use the hair dryer to heat it like cdaddy said, with a piece of thin string, like fishing line or something to get under the sticker and swork it through sticker. You'll more than likely have goo on the tank afterwards, but it'll come off with some washing and wiping.
Goo Gone should work. Try it on the underside of the tank in a hidden spot where it won't be seen. It has Xylene in it which might streak the paint if it's older or faded.
If the paint is good, it won't affect it any.
If you use a heatgun and have any glue on the tank, WD40 and rubbing alcohol works great for removing it, but not at the same time.