I have a 75 US gallon on the first floor above a full basement (so the same as if it's a second story). I did some research and talked to a structural engineer who advised the following.
- The tank should span several floor joists (so the tank should be perpendicular to the floor joists)
- The Tank should be placed along a load bearing wall (as there would also be a load bearing wall or beam under that wall)
- The tank should be placed in a room with a smaller span from support to support. Meaning that if you have a room with a 16 foot span and one with a 10 foot span between supports, put it in the room with the 10 foot span as the structure can take more load due to the lesser span/more support.
Obviously, there are many additional variables to consider such as age of home (different building code standards over time), size of supporting joists under the floor, have the beams been compromised in any way (such as holed drilled to run wires,etc.), etc.
Fully loaded my 75 US gallon is estimated to be around 900 pounds per the information that came with the tank and stand. Since mine is over the basement, I also have the option to add additional support from below. We haven't seen any bowing or distortion of the floor supports, we placed the tank perpendicular to the floor supports, along a load bearing wall, in a room that's about 12 ' x 12'.
Edited to add: One more thing, the engineer advised that the stand should NOT be a stand with 4 legs, but one that had several continual supports across the bottom to help spread the load evenly. If you flip my stand over and look at the bottom, in addition to the solid bottom and two solid sides, there are three continial support pieces running from one side to the other (side to side, not front to back). This balances the load evenly.
Should've read all the OP first so I wouldn't have to keep editing
The OP point about large people or filled tubs being temporary weight vs. permanent weight of a fish tank are very important. The structural engineer made that very point. Floors are made with a +x safety factor for temporary weight, permanent weight however has a continuous impact on load and the two should not be considered the same. Continual load is much different than tranisent load.