I'm not sure we'd want an actual sphere. Maybe a cylinder with rounded edges. The problem is that if the floor under the storage tank moves at all -- e.g. earthquake or subsidence -- the tank is probably going to try to move. Since it will likely weigh 500kg or more, typical interior walls -- at least in North America -- probably won't constrain its motion much, We'd probably prefer that our hot water storage tanks stayed put.
My parents had one of those big tanks as part of a solar hot water installation. Per code -- doubtless written with typical tall, thin domestic hot water tanks in mind, that storage tank was securely strapped to an interior wall. It was pretty clear that if that tank decided to move, the wall was going traveling with it.