I've heard a rumor that PPM levels in an aero setup should stay approximately the same over time. If the ppm levels rise or fall, this mean the target value is off. If your ppm levels are rising, this means that the plants are sucking up more water than nutrients, and the target ppm level should be decreased. If the ppm levels are falling, this means the plants are sucking up more nutrients than water, and your target ppm level should be increased. I thought about this a great deal, but still cannot determine if this a good rule to follow. If it is, does it apply to all stages of growth (except the beginning when you start out with 0 ppm usually)? I ask this because my plants right now are in heavy flowering, and they are sucking up way more water, (ppm levels are rising constantly) so maybe I should start backing down on the nutes (lower the ppm)?