That depends on what the floor is made of and the walls/beams supporting it.
In South Africa and Dubai and the fishy people I have had the opportunity to meet who live in apts/keep tanks upstairs, space seems to be more a limit than weight. The most memorable here in Dubai was one guy who I sold my Jardini Aro to a year or two ago had 2 x tanks each one with dimensions of 2m x 90cm x 90cm , and one standard 180gal (180cm x 60cm x 60cm) and he lived in a one bed apt, on the 3rd floor. I asked him if it ever worried him (because thats alot of water), and he said the thought never occurred to him. He just always did this.
Maybe this is largely due to the fact that in SA houses are brick/concrete and in Dubai its all cement/concrete. Reinforced concrete beams and solid walls (supports).
I have never been to the US before, but it seems like most upstairs areas are made of wood?