If your tap water doesn't have nitrates or other negatives, then yes, adding back a bit of tap is common. Always depends on what's in your tap. I think discus buffers are trying to add back some KH, carbonate, which helps stabilize pH, but that's often what's in hard tap water. I think all animals need some sodium, calcium, magnesium, and potassium, the basic electrolyte elements. But how much varies.