My owners manual has this warning;
"IMPORTANT: never fill the spa with soft water unless an
appropriate mineral supplement is immediately added.
If your water is extremely hard, it is preferable to either dilute the water’s hardness by blending the water with water from a water
softener, or by the addition of a special water softening
chemical (see your authorized Bullfrog Spa dealer)."
I think I read somewhere that if the water is void of minerals it wiil try to draw minerals from the pumps, plumbing, fittings etc...