I don't think the danger is that the tank could actually fall through the floor, but that there could be movement or flexing of the floor without proper reinforcement.
Shifting or bowing of the flooring, while not causing the tank to fall through, can cause it to split a seam, crack, or tip over (as Bud8Fan eluded to, kids).
Burt
Shifting or bowing of the flooring, while not causing the tank to fall through, can cause it to split a seam, crack, or tip over (as Bud8Fan eluded to, kids).
Burt