Elevated levels of magnesium and calcium in water can make it taste smoother or softer.

“In fact, many people find that with elevated levels of magnesium and calcium and water, it actually tastes smoother or softer in their mouth. So hard water tastes smooth or soft. I know it's all very counterintuitive, but I think it's important to point this out because a number of times you'll hear or read about filtering water so that it tastes smoother and better. And oftentimes that's happening because the "hardness" of water that is the concentrations of magnesium and calcium are actually increasing.”