Tap water is never stripped of minerals. I suppose it's possible SOMEWHERE, but I've never heard of it happening anywhere, and it seems too expensive and pointless to make such a thing even remotely likely.
The vast majority of the time, the problem is that the mineral content of tap water is TOO HIGH, to the point where many people instead choose to buy distilled or reverse osmosis water (which has no mineral content, aka demineralized), and either use it to dilute their tap water (lowering mineral concentrations) or by itself with NO tap water at all, which requires adding minerals manually.
I dilute my tap water with RO water to about 25% (1:3, tap:RO), and do use a small amount of (USUALLY) calcium chloride and/or gypsum. But if you don't know the mineral content of your tap water (especially if you're not diluting with demineralized water), then you're better off not messing with the water chemistry. It is far easier to make things worse with brewing salts, than it is to improve a beer. You really need to know what you're starting off with in order to make correct decisions about the type and amount of minerals to add.
