That is a very unusual design for a home floor joist. In theory it should be much stronger than a typical floor joist. That is making the assumption that it was designed and installed correctly which is something I can't comment on.
I agree with calling in an engineer and having them review it. You can easily and cheaply run new beam from the foundation to a jack running perpendicular to those joists and you should be fine. If you can't get it to the top of the foundation, or if that part of the foundation isn't load bearing, you may need a jack on each end of each support.
This really is unusual enough to make calling in a structural engineer worth while. Even a 75g tank could end up being questionable. Those still weigh around 1000 pounds if you don't use a sump.