Well, without getting into "stoichiometry" or electromotive force and all that rot [god forbid the brain damage], I'm left wondering why HCl would attack a carbonate but not a sulfate of Ca+. I know our soils here are loaded with gypsum, which Wiki tells me is Calcium Sulfate CaSO4 - and which I presumed was the bulk of the white deposit on my tile simply because of its dominant presence. But maybe it's carbonate because it fizzed up so well when removing. It's also likely we have much epsom salt, Magnesium sulfate in our soils, but more must be learned.
At risk of diverting attention down darker alleyways, this thread gives me chance to raise subject of filling a pool with >> softened << water. Has anyone actually done this? I speak about it occasionally at the pool stores (NPS), and once the sales guys get over their shock, they quickly begin avoiding me like a raving lunatic. Honest boys, I'm not dangerous.... I just like to think out of the box.!!
So, I ask Community for any feedback on swimming pools filled with softened water.
Just recently, thanks to a TDS meter, I just discovered that my softened household water actually has a higher TDS reading than the source well. That it doesn't leave spots then, and makes for better cleaning, suggests its salts (Lithium in my case) are far more soluble and therefore less likely to create white deposit at waterline and splash zones. It might make for a superior swimming experience too, plus delay the cost of drain and refill (about $700 here in Vegas) which most locals do every couple years since our utility water is very hard 290ppm. But I cannot find any definitive resource to report on the idea.
BTW - I have spoken with many people about RO and it just isn't feasible in any case, much less with a home on well and septic tank. FYI.