I have a 350 gal. on a stilt house built it 1920. All I did was buy cement blocks, and floor jacks from Lowes and supported the floor. I have 6 jacks under the tank. I have not had one problem. I also have a 125 and 150 with no supports and did have a 180 with no supports.
If there is any concern just reinforce the floor. Its a $100 or so well spent.