Water out of the tap aluminum, arsenic, copper, iron, lead, pesticides, herbicides, uranium, and more normally has a bunch of things in it, including calcium, magnesium, potassium, iron, copper and a bunch of nasty's like aluminum, arsenic, pesticides, herbicides, uranium, etc., etc............ a normal reverse osmosis device (mine has 4 stages) does a good job of stripping all these out of the water. My pre-device water has a ppm of around 240, after the device it comes out around 10-20 ppm.
So tap water will have some Cal/Mag in it, but not after running it thru the device. My plants are showing more Cal/Mag deficiencies now because it's being stripped out of the water I'm watering with and I'm not adding any of that the last 2 weeks. People who use these devices normally have to add 5-10ml per gallon of Cal/Mag to ward off any deficiencies like tacoing.