My 2 cents. When I was putting everything together & realized the investment in my tank, I added a little more to my budget. A carpenter got under my house & used mobile home jacks & cinder blocks in a couple of spots.
I might never have needed it, but it is one less thing to worry about. For the $100 it cost me, it was well worth it.
I figure my 120 weighs no more than 1800 lbs
Will the tank be located on a load bearing wall? YES
Perpendicular to floor joists? YES
Spacing of said floor joists? 12 INCHES
Construction of same? 2x12 CYPRESS
Age of building? 68 YEARS