I found the same problem (well not really a problem)....in my first grow they drank more water....left more nutes. Second grow....ate more nutes....ppm's went down instead of up. This grow is the same. I'm figuring it's the time of year. My first grow was in summer.....AC on constantly. Next grow in fall and now in winter.....I'm actually seeing it stay more even in this grow. Could be coincidence but.....??