The key is making sure it is level. I have a 65 gallon with a 38 gallon sump on my 2nd floor and its fine, mine actually only sits halfway on 1 floor joist and hasnt been a problem for over a year. If possible try to figure out which way your floor joists run and try to sit the tank on at least 2, otherwise you might notice some floor sagging and the tank might worry you a little like mine does.