"Many small tanks clustered together to get the same volume as one large tank bought you nothing."

How about trying something like my "Pipelauncher" idea- pipes that make a bigger pipe. Or if you want a spherical shape a large sphere constructed from smaller hemispheres.
Actually the reason why I think using pipes to make a larger pipe would be a good idea is in order to increase strength of the structure [more for reasons of dealing with any internal or external pressures (from gas or ocean pressure) because these pressures are mostly balanced with only pressure differences of around 100 psi- even though pipelauncher could be "operating" in a thousand feet of water (1000' of water equals around 450 psi)].
BUT a bundle of pipes welded together with a hollow center which also holds pressure should hold more volume at higher pressure than a single walled large pipe (cylindrical tank). I was at one point going to use this idea for making a "small rocket"- until I learned that my idea of a small rocket, wasn't a small rocket:).