How are you determining that your ppm are at 15 -20 ppm? You should have no trouble getting that info from your local water plant. Call there and ask for the average ph and total hardness. It may fluctuate seasonally a little but you should get a reliable base line and the numbers will be based on chemical titration and lab grade probes as opposed to grow store ppm pens. 15 -20 ppm is extremely soft for tap water. If it truly is that soft I'm guessing it is RO or nano (membrane).