the pro-mix is not killing them and you are probably seeing the dirt itself on your meter. pro-mix has nothing in it but some peat and a shit load of perlite.the parts per million(3000-4500) you are reading is the mix itself and you would need an ec meter to read the actual water coming out of your runoff .i use the pro-mix bx from start to finish,good stuff. i do my clones in it and add cal-mag @ 300 ppm because i use r/o water and during flower i run them @700-950 ppm for every watering until flush.it's considered a soilless mix kinda like growing with hydro,you gotta add something to it after about the 3rd week of veg or your plants will go downhill fast.what kind of water are you using?maybe it has too much iron,cal ,or mag in it.your ph meter will work for the runoff but not the ppm meter.gl.pro mix (orange) is killing my plants. I put no nutes and the ppm comes out at between 3000 and 4500 ppm