This comes from experience. It works...HOWEVER, you don't want to do this on anything with paint on it. A old bodyshop owner I used to hang with used to do with on classic cars that he would strip down to bare metal. It popped the dents out and lessened the amount of body filler he would have to use. BUT, it is still a skill and not something I recommend everyone try. Also, if you try it on painted metal you will one, burn your paint because you really need a torch to get the heat to pop it out and two you will crack/flake the paint.
I recommend taking the tank to a PDR (Paintless dent Repair) person and seeing if they could finess it out for you.