Last night I googled second floor load pressure rating, or something along those lines, and surprisingly there were a bunch of aquarium conversations in the mix. I don't have the time to look it up, but there was one thread started by a structural engineer who set out to try to be informative as possible. It was quite interesting, the OP might want to try to find it. The biggest takeaway I got out of it was that the stress from weight on the floor can/will increase over time... just because a floor rated at 30-40 psi/sqft (apparently that is what residential floors are rated for, lots of considerations go into that number) can handle much more than that at any given time, over the course of time that stress continues to have more and more affect on the structure.
It might be worthwhile getting a structural engineer involved in this type of project to protect your investment (not just the tank).