If the water goes down and the PPM's are going up your probably over feeding the reservoir and they dont grow, lol. My theory on PH drift is the feed mix sets the PH and different nutrients and supplements raise and lower the PH so as the plant consumes more of one than the other the PH changes. That could be total BS but my auto feed and ebb & flow reservoirs stay set if anything they climb a little. The RDWC is a different story, I will use a gallon of PH UP in a grow but its on a PH dosier