More Information so you can counsel me properly
I removed some of the ceiling tiles in the finished basement to inspect the joists. Sure enough they are 16" apart and 2 x 10" joists. HOWEVER there is a ray of hope! I found a "bearing wall" that connects to the parallels joists perpendicularly. This wall has two 2 x 10" joists running perpendicular to the "parallel joists". These joists are immediately beside each other and touching, so the effect would be I assume of a 4" x 10" joist in total for support running perpendicular to the floor joists.
So I could "straddle" the junction of the 2 beams x 10" bearing wall and the parallel joists that are 16" apart. This to me seems like it should hold a 90 gallon tank long term without sagging. But what do you folks think?