Tile the floor area under the tank coming out a little around for aesthetics. You could also put in a vent fan to the outside behind the tank to help ventilate. Or a dehumidifier hooked to a drain.
I've always been told by structural engineers that you use the sq/ft load of a refrigerator for a comparison to your tank/hottub. If your fridge isn't sagging your floor neither will the tank if it's the same or less per sq/ft load.
Sent from my iPhone using Tapatalk