[Myka]

That would really depend on the structural soundness of the house. If it is an older house I would question it more. If you're placing the tank perpendicular to the floor joints I wouldn't worry as much as I would if it had to go parallel to the joists.

To try to answer your question, in an ideal situation where it house is relatively new (say 15 years), and the tank is sitting perpendicular to the floor joists I wouldn't worry about the tank's weight until I start to approach 350 gallons or so. Even that I think would be on the safe side.