you might want to ask your landlord anyway; better than waking up that people underneath or the landlord blame you for the floor / their ceiling bowing, cracking or whatever else. some of the old houses are just not designed to support such a load; a tank will weigh at ~ 10 pounds for every gallon of total capacity. if you prefer not to ask at least make sure the tank is as close as possible to major structural members like the outside frame, the middle (load carrying) beam, etc. I lived in a room where the neighbor upstairs had a walk around tank, the ceiling always looked suspect to me (concave) and would I have known I would have raised hell for having to sleep with a 1000 pound tank on top of my head and the frame bowing under the load, ....
things changed meanwhile but I would still caution against anything that could get you in trouble.