leaves sagging

Hi everybody, I am currently growing 2 photoperiod plants, my medium is coco with some perlite. The first 2 feedings i used synthetic nutrients because i still had some left, but now I want to switch to a complete organic regiment. I flushed the two plants with water ph'ed to 6.2, and that is...

I got 2 hydroponics. They both feeding from the same reservoir, and they both growing well. But one of the plant start sagging straight after the light is switched off. When the light is back on, the plant also recovers. Is it normal? Or am I doing something wrong? thanks for advice Here's...