Re: DIY tank... even if you can't weld
When they do HYDRO STATIC testing on compressed air tanks (like SCUBA) they fill with water as it is incompressible essentially.
The tanks are measure prior, while, and after. They are supposed to only stretch the diameter (or something) and then go back close to the original measurement. They pass if go don't stretch too much and go close to the original enough, otherwise fail.
They probably also have the tank already dunked in another open bath tank of water in case it gives way to stop some of the exploding force.
Filling of SCUBA tanks with air, they get hot due to the compression, so they also should have them in water bath for both filling better and safety. Otherwise filling very slowly does it, but still with out the water bath there is more for risk if burst.
Maybe the metal being formed this way could also be submerged in a open tank of water, just in case the whole seam rips or shrapnel is formed.
Probably would have heard of accidents, but maybe not?