So you may and may not depending on the makeup... like I say in my original post PH is not really something you can use on its own.
You can look up your water report or post it here and I can give you the low down on it.
But to answer typically the ratio of calcium to Mg is not adequate in tap water... most times you will find adequate calcium but very much lacking magnesium and as we know the nutrient ratios are extremely important to maintain. So in this case instead of adding cal mag you only need to add Mg to bring the ratio back. Ideally about 100ppm Cal and 50ppm of Mg. In most cases if you ppm from tap is between 100-200ppm simply adding 1 gram per gal of magnesium sulfate (epsom salt) will bring the ratios back into a favorable place.