I use tap water and because of this I add very little calmag, but I just used some at 30 days in flower...
My tap water comes out at about 335ppm, so not to bad really.. For those who grow on a small scale and only use like 2gal of water every 2 to 3 days, try investing in a britta pitcher.. filters last a good while compared to more expensive ones..
However, like 2broke, I too just fill the water jugs and allow them to sit with lids off so chlorine and such can dissipate.. as long as my plants look healthy I don't fret to much...
Note: Most pool supply stores will check your water for free..
Also, never mix your nutrients together raw, then into water... I add mine to a smaller mixing cup with water in it, then pour that into my gallon jug.. mixing them together in water is fine, but never raw with one another as they tend to break each other down when done like that.. peace