In my case, the tank, stand and contents weigh about 4,000 pounds. Tank sits on a first floor (10" joists), but there are jack posts between the floor and basement slab on all four corners. I would not keep a large tank without full support all the way down to slab. FWIW, I agree that spectacular floor collapse is highly unlikely, except in extreme circumstances. What is much more likely, and the thing to watch out for, is gradual floor deflection that puts unnecessary strain on the tank resulting in premature failure.