A lot depends on what the 2nd floor is made of. If it is an attic conversion, I might be leery - since the attic was not meant to support a lot of weight originally. Otherwise, you need ot find out how the floor was constructed to get an idea of the weight bearing capacity, and also to properly locate the tank over a load bearing wall & across the joists.
A cautionary tale - My house, 60 yr old, piano ~1000lb on main floor ... floor sagged over 3/4" in the span of 6-9 months ..... Your tank may be 500-600lb, but you get the idea.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.