If your tank is glass and if the credit card doesn't work, you can try one of those clickable paint/wallpaper scraper knives. They are also used to remove stickers from car windows. I am an artist (oils) and use them as palette scrapers since I mix paint on a glass palette. Just be sure that you put a clean razor blade in it and that there aren't paints or chemicals on the knife. If you aren't familiar with what I am describing I can send you a pic. That's what I use, in addition to vinegar, and it works like a charm and leaves no scratches. This way you won't have to fool with muratic acid. Good luck!