I think you're right to be concerned about the weight on a second floor. While, yes, you can add up all the weight of furniture in your living room, but do you think that would be a wise idea to put all that weight in one spot? The reason your furniture isn't a problem is because the weight is distributed over a larger area. Consider also that you are now going to ADD the weight of the tank and it's contents to the weight of the furniture you already have.
scubabum is right though, if you're going to put a rediculous amount of weight on an upper floor, then near an outer perimiter wall is the best place to put it.
Now, for the matter of leaks/spills/malfunctions: a second floor apartment, I would go no larger than a 55 personally, and even at that, if I were to spring a leak at some point, I'd definitely have renter's insurance that would cover it if something were to happen and your downstairs neighbors are suddenly taking a salt-water shower. They would probably not be pleased, nor would the apartment owners, and for some reason, words that spring to mind are: eviction; lawsuit; ***-kicking, etc.
I know if I had upstairs neighbors that ruined all my stuff and my apartment with an unexpected leak in a fish tank, I'd pound them senseless.
I'm sure you're a responsible fish keeper though, that would never put someone else through that against their will.
Remember, no matter how good you are, or how meticulous you maintain things, accidents and unexpected equipment failures will happen.
Just my thoughts on the matter.
Good luck!
