Just got a new spa a couple weeks ago, and Spa Sentry came in the starter pack. We have very soft water here already (test strip showed CH of tap water as indistinguishable from 0), and the spa store guy suggested using this instead of bringing the CH up. Apparently it's supposed to protect the heater etc. in an alternative way, so increased hardness isn't necessary. It also stabilizes the PH.
Just wondering if anyone has any knowledge or opinions about this. Will it indeed protect the spa's components from the soft water? If so, I guess there's no harm; the only reason to not use it (and instead raise the CH) on the next fill would be cost? In either case I'll be adding borates (currently with SpaGuard Optimizer Plus, which also came with the starter pack; in the future probably with boric acid and/or borax).