Your husband is correct about the hard tap water. It will not damage the plaster. Since the water is hard and will eventually get too hard, you could rent a typical water softener (for homes) and run it for part of the filling process. That can be a cheaper way to go to soften the tap water.
Soft water is the concern, so don't lower the CH below 250 ppm, which probably means using the soft water unit during the last third of the fill. Good luck.
Soft water is the concern, so don't lower the CH below 250 ppm, which probably means using the soft water unit during the last third of the fill. Good luck.