I have a 75 gal
on what is basicly the second floor since there is a finished basement under it. If the floor is properly supported you could put any size tank you wanted. When in doubt try to verify the size and spacing of the floor joists and then work from there. IMO
most standard houses built to modern housing code should be able to support a 90gal tank on a above ground floor if the tank is placed close to a load bearing wall.
Disclamer: I am not a contractor or carpenter.