You can remove pretty much any dent with a stud welder and puller. Back in the old days, they would drill a bunch of holes and use a puller with a screw. Not they use a stud welder and puller.
I restore classic cars as a side hobby and have the tools for this.
The paintless uses tools to push, but you need more to "re bend" the steel.
The problem with stud welding is that you have to make contact with the metal, so you will be repainting the tank.
I see two options, try again with suction or glued and bending. I'm guessing the paintless guy had what looks like long specialized wires. You can also glue on an attachment to a puller. Several glue points and some good pulling can do the trick. You can also use dry ice, heat and air pressure inside the tank. Adding a bit of air pressure inside the tank while pulling on the glued pullers could do the trick.
Once it's folded, it's damn hard to restore without removing the paint.
When/if you do use any filler, make sure you use a little as you can, pull the dent with a stud puller and you can get it very, very close, then just a thin coating, then block it down and repaint.
Another option it to just leave it. It's a war wound and she still lives... You have a few scars on you as well