A quick google search would bring up an engineers report on the matter. My brother looked into it when he wanted to put his 125 on the second floor of his house. Apartments do have higher requirements than homes. The tank should be placed against a load bearing wall. For added support make sure the tank is perpendicular to the floor joists. There isn't really any normal tank size that will "fall through" the floor but as noted above the joists may start to sag over time.