Tap water will have calcium and magnesium as well as a whole host of other crap. Chlorine fluoride lots of stuff at that level. Measuring with a tds meter which is the same thing as EC meter. Take the EC and times it by 500 or 700 depending on what meter you use.
I've been adding calmag to my water and feedings b/c of the LED's. It helps, so I guess it's safe to assume that I don't need to concern myself with that reading at this point. I guess it must me low where I live.