A few years back Coca-Cola got in trouble by doing exactly this. What they were doing was taking tap-water, and purifying it so they could sell it as "Dasani" -

A few years back Coca-Cola got in trouble by doing exactly this. What they were doing was taking tap-water, and purifying it so they could sell it as "Dasani" - not a mineral water product, exactly, but a "lifestyle drink" they called it. The thing about water when it's pure enough is that it's remarkably odd. It's unpleasant stuff: it'll weaken concrete if it leaks onto it, apparently it'll damage brass and steel, and it'll leach the minerals out of your teeth, but only until the water is no longer pure enough to do so. So what people do is, as you say, add buffer minerals back into the purified water to take the edge off. What Coca-Cola got wrong was the dosing: they put ten times the amount of buffer minerals into the water as they should have, and made it toxic. So they'd taken a perfectly safe tap-water supply and ruined it.