Hi Ricky ,It"s water that has been cleaned with a reverse osomosis filtration system It removes contaminants from the water and that way you dont have ppm's messing with nutrient concentration. And pure water is always the best. Organic growers never use tap water it kills beneficial bacteria due to chlorine!

ppm means particals per million. for 1000000 particals of water their is the amount of contaminant 1000000/1=1 ppm 1000000/155= 155 ppm