I take it this tank is glass? Then try below methods.
Single edge razor with a little(carefully applied) rubbing alcohol should work fine for the sticker. Cap the rubbing alcohol bottle with a wash cloth and apply, then rub the cloth all over the sticker without getting any in the tank. This method takes some rubbing but don't give up.
or to loosen up the sticker first use a hair dryer on high to soften the glue, then scrape before the glue cools down.