If you are growing using soil, you can pretty much get away with any water that is PH balanced.. 5-7....

If you want to be perfect... you need water that starts (before you add nuts) at PPM 50 or less.

You will need to purchase a Parts Per Million meter.....

and you will set the PH at 6.5-6.8..

If you are growing hydro or aeroponic you will need water that is 50 or less PPM.

Soil is very forgiving and you can get away with murder...

: )