Depends on the size and shape of the dent, and whether there are hard bends at the edges. Also on whether the tank is easily replaced
I've had good luck using push rods made out of 3/4 inch or bigger steel rod with the ends rounded and bent to shape can reach most areas on a tank and let you push them out.
Also welding some disks to some thinner rod bent into eyes, or grind the heads of some carriage bolts flat, then solder them to the tank in the center of the dent and use them as puller posts, keep pressure on them and tap around the edge of the dent, that will normally walk the dent out.
Have heard of using the same pullers with super glue to pull dents and avoiding the solder aspect, haven't tried it but plan to experiment. Need to have a larger head on them though to increase surface area.
Ken.