Does anyone have a good idea how to securely hang tanks?
kr2y5,
Can you purchase a relatively short piece (~6 ft) of scrap steel I-beam, suspend it from and close to the top of your garage ceiling with two pieces of chain, and purchase/erect a block-and-tackle on rollers which you attach to the I beam. You could then use this system to lift your doubles up from ground level, roll over your stock tank, and lower your doubles down inside the stock tank.
Alternately, you might acquire one of those manually-operated devices used for lifting engines out of the engine bays of automobiles. They are built on heavy duty castors and have a block-and-tackle as a major component.
Alternately, you can switch to PST HP (3,500 psig) 100's for doubles. Even cave-filled, these are not too heavy to lift by hand.
Or you can do as I did a few years ago: Break apart your PST HP (3,500 psig) 120 doubles, and never look back. (My back has thanked me ever since!)
Safe Diving,
rx7diver