I say under 80 may be safest since I was told most bathtubs can hold about 80 gallons(but the floor is braced for tubs), I am renting too and I ask this all the time, but there is no definite answer it all depends where you live and how the building is made, my rent contract says nothing over 30 gallons I think, but I asked the managers office and they said no pets at all, which is complete bs since half the people here have something anyway, What I finally decided was "screw it! " Why the heck do I need a gigantic fish tank in a small place thats not even mine? So I am just gonna be patient ,keep some smaller tanks (unkown by the managers) and just wait until I have my own house to unload Monster style, Jerry Springers final thought....... If someone living in a second story apartment really wants a bigger tank that bad , how hard can it be to just move into a ground floor apartment !!!!?????? AND, if you want a big tank upstairs in your own home and it worries you at all call a pro to come out and check your unique house with itsown design etc. that may not be like anyone elses, I can say that this is the conclusion to my own little adventure in second floor Monster tanks! Don't get me wrong I want a Monster tank bad! very bad!!! Good luck to all my second floor fishkeeping brothers and sisters!!!!!