Late to the question here but some points that I like might be worth mention. Much depends on how the space under the floor is built. On concrete like a basement, it is much easier as the solid, flat surface is there but if it is a crawlspace or dirt, I like to leave jacks in place that can be adjusted as the dirt settles or moves.
For dirt like in crawlspaces, I lay down a Treated 2X12, use the jack posts and then top it with a doubled 2X6/8 on top laid on edge.
Tanks are not really as hard to hold up as we often read about, even in old houses. I think of it like having a group of fat friends come over. If worried about the floor collapsing when they flop on the couch, I worry about putting a tank there but in most houses, that is not a big worry.
If the floor begins to sag, it is not a sudden thing. Just something to fix if you see the water level begin to go off a bit too much. Better to fix first but not a big thing if you fix it later, either. Just don't wait to never fix it?