generaly speaking, 200 ppm is about as high as you want your water to start at.. I think that just ppm isn't enough info though. You really want to know what is IN the tap water... Get a water report and check the level of sodium and such. also, see if your water department uses Chloramine (chlorine & amonia). If you have that, you may want to run your water through a scrubber with a KDF filter....