first thing to do is to cut solid bridging (if the floor joists are 2x8 then use them). cut the 2x's to fit perpindicular between the existing joists snugly (should be 14 1/2"). This will help tie the floor together. The bridging should be installed under where the tank sits, and maybe a little more. so if the tank is 48" use 50 -60 " of bridging. then I would nail a 2x to the bottom of the floor joist where you are going to put the posts. Then either put about 4" of concrete in the crawl space extending a foot from where the posts will sit, and put 4x4 posts. It getting concrete in there is too tough use 4" solid concrete blocks. One for each corner. dig down slightly to make a level surface for the blocks and put the posts on them.
When you install the posts measure the length you need and cut the posts to length, then use a jack to SLOWLY move the floor up just enough to install the posts. hth