Don't use steel wool. Give a new razor blade a try. Also soak some towels in warm vinegar and drape over tank for few hours before trying to scrape it off.
i took calcium deposite off a tank with a sander. its the portable sander you hold on one hand and you gonna use the buffing pad then use toothpaste, try to get the gritty toothpaste the one with baking soda in it. then buff the glass with it using then sander . remember you are using the buffer pad not sandpaper....the toothpaste acts as a polishing agent. worked wonders on a used tank i got a couple years ago...came out like new