So many uses for scuba tanks, lets add;
I made a TV table using four scuba tanks as legs,
I rigged a regulator to attach an air hose for tools, so I have portable air tools. The air ratchet is really handy when hanging upside down into a marine engine compartment.
I painted a couple of cars using scuba tanks as an air source. Ten tanks will give you enough air to put two coats of paint on a large car.
Scuba tanks make great rollers for moving heavy machinery, when my compressor was dropped off in the parking lot, with impending rain, I levered one end up and slid a scuba tank under it. By pushing and levering I got four scuba tanks under the length of the compressor, and I was able to roll the compressor indoors by myself, taking the tank that the compressor had just rolled off and putting it to the front, repeat as necessary.
I am thinking about using a bunch of them to make a giant wind chime