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Have you tried 1/4 cup of Stain and Scale or Metal Gone on initial fill adding nothing else for 24hrs. Then Balance and shock. Use weekly maintenance dose of stain and scale or metal gone-2-4 capfuls.
Well, sheesh, why do you even need to concern yourself with any of that. The way everybody here hollers about the superiority of Hot Spring no by pass filtration, ALL of that stuff will get filtered out in just a few short hours and you'll have sparkling spring water flowing through your jets, at least thats the impression that's given round here.
But Chas, why would such a thing ever be needed in a HotSpring Spa?