First of all, I would recommend that you get "renters" insurance. I live in a second story suite, that I own, but still have an add-on to my insurance policy for my aquarium. ($10.00) every six months. Pretty cheap.
Consider that a 100 gal tank full of water only, will weigh over
800lbs. No telling about the rocks, you'd just have to weigh them also. Also, does your apartment have a concrete underfloor? or is it just wood beams? How old and what shape is the apartment building in, generally. Actually, that weight would be distributed enough, that in a fairly new, modern apartment, you shouldn't have any problem, as long as you place it near a wall. Check your lease, Some (few) but some apartments have policies about aquariums and water beds. I would be more worried about the aquarium breaking (unlikely) and causing serious water damage, than I would, it falling through a floor; get (if you don't already have) renters insurance. If is an older home converted into apartments, or if the overall construction of the building is questionable, maybe you should think smaller. Maybe a 55gal. I have never actually heard of an aquarium falling through a floor, but I'm sure it has happened at least once, somewhere. Again, I'd be more concerned about a slow leak developing while you were on vacation or something, and causing serious water damage. I't happens. If you can find out from your landlord, if you have a concrete under-floor, I wouldn't worry about the size at all.