I will let big boys jump in here and correct me if I'm wrong.
I was researching soft water and spas. Most spas will tell you not to fill the spa with soft water becaue the minerals taken out of the water create a void that naturally will want to be filled. So if you fill your spa with soft water it will begin pulling minerals from your spa, be that from the heater, pump, ect. If you have spa that has a problem because water chemistry was not balanced...well you might need to replace your pump. That could be the start of it...
This is also from a guy who doesn't have a spa yet..., the key word is yet!