Pure water is ideal, but may not be worth the cost for some people. As an example of what you can learn by consulting the local water authority, here's my tap water:
nitrogen 3 ppm
phosphorous 2.4 ppm
potassium 3 ppm
Other notables:
sulfate 44 ppm
chloride 42 ppm
calcium 38 ppm
sodium 27 ppm
magnesium 8 ppm
iron .07 ppm
Other things in much smaller portions.
Some people have much worse water than this coming out of their tap.
nitrogen 3 ppm
phosphorous 2.4 ppm
potassium 3 ppm
Other notables:
sulfate 44 ppm
chloride 42 ppm
calcium 38 ppm
sodium 27 ppm
magnesium 8 ppm
iron .07 ppm
Other things in much smaller portions.
Some people have much worse water than this coming out of their tap.