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Taken from TDS thread...In my expereince, it is impossible to give people a "timeline" on water longevity unless you have a clear understanding on the source water, potential usage of the spa and its initial TDS reading. If your TDS starts off at 150, sure you may get close to 6 months (which is pushing the envelope a bit IMO), but if your TDS is starting off at 800, you may only get 2-3 months (if that). We had many people on well water that had initial readings of 1000+ TDS that only got 4-6 weeks out of a fresh fill. The best analogy for TDS is a coffee cup. You can keep adding sugar to it but at some point the coffee becomes saturated and the sugar will not dissolve and instead, sit in the bottom of the cup. Anything past 1500 TDS in a spa prevents your chemicals from properly doing their job as the water is now becoming saturated essencially throwing good money after bad. The mistake many people make is that they base their water quality off of clarity. Battery acid is also clear...