Well...I have a 150G, 100G, 55G, 15G in my 2nd floor bedroom. My 150G sits perp across several joists but its dead in the middle of the 2nd floor against a non-load bearing wall. I would say if your home is well constructed the tank should be hitting atleast 2 joists in the least it should be fine. Is the tank against a load bearing wall? I used to house 2*55G tanks on a dual iron stand...same gallonage, smaller foot pring for distribution...and running parallel with the joists...it was hitting 1 joist and also not on a load bearing wall. Well constructed homes you really shouldnt worry about your tanks until you get into the several hundred gallon spectrum IMO.