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I know my manual said not to use softened water... so does metal gone stuff maybe help counter the soft water? I don't know anything... just guessing/rambling.
Using a water softer should do some of what the metal gon would do, so they are more the same than opposites. If you used the softened water you will most likely just have to add enough calcium hardness to compinsate. Shouldnt be to big a deal, and your dealer can test for your calcium levels. Next time you might want to use the unsoftened water or you will just have to add a bunch of calcium each time. Congratulations on the spa!