What kind of floor are you thinking of using?

I believe the floor upstairs is wooden beams, around 12-16" apart. I know the tank will go accross these, and not run along them if that makes much difference?

In a newly constructed house the dead loading requirements on the second floor should be about 80-100# psf. That would suggest the weight being distributed over about 25 SF. It will be less likely to cause defelection being next to a bearing wall. If you excede that loading build a 2 x 10 platform to distribute the weight across several joists. A steel plate would do the same job. Loading this much weight on one or two joists could cause a structural failure.