There is no way for anyone to make an all inclusive statement whether or not it is safe for everyone. It would require an inspection of the structure.
The best way for you to install any tank (that's not on a concrete slab) would be to put it against an outside wall, or an interior wall that is a bearing wall (has structural support underneath it). The tank should always run perpendicular to the floor joists.
As you can see, without doing a physical site inspection it would be impossible to determine if your tank is safe on the existing structure.
A 90 gallon tank SHOULD be safe though. Not definitely safe, but it SHOULD be ok.
Wish I could be of more definitive help but it's just not possible.
swimmer