Hello; This makes a lot of assumptions that cannot be applied to all floors. There simply is no way to know for sure without checking the condition of the structure. For an example several of the floor joists in my current house were damaged. Deep notches were cut into the lower portion of the joists right in the center of the span. I had to sister in new joists and this was in a bedroom.I was told any floor can handle standard 125 gallon tanks, but if moving to more then 150 re-enforcement is required. Is that rule for first floors only?
There can also be rot or insect damage. Even new houses can be built poorly as seen in some of the Mike Holmes TV shows.
I must say the advice of a 125 is safe on any floor to be poor.