Steel or aluminum? I can tell you what I’ve done recently with 6 gal tanks:
Pint of acetone sloshed around for a couple of days to dissolve polymerized gas/oil stuff.
Hot water, TSP, and pint of stones shaken and sloshed around for a good while.
Rinse.
Tread with phosphoric acid solution such as Phospos or similar.
Any actual holes patched with epoxy.
Assuming inspection shows rust/scale gone and metal a clean grayish color.
Coat inside with POR15.
All this, of course, is a pain in the ass, not including refinishing the outside of the tank, making gaskets, and all that. Is there one cleaner that can do much of this in one treatment? If so, I would like to know about it.