How'd pale,. I ant no hydro person but cannabis plants do tend to yellow even if uve feed em well, toward the end of flowering. Im a soil grower myself and there nothing too much too be concerned about, the plant will take from the fan leafs if needed. As you said yourself
the other plants dont show signs of it each plant is different how much longer do you intend to flower em, 2 ,3 more weeks if they are indica plants yellowing at week six ant a bigie..
the yellowing you describe would have me too concerned if they been good til now..

Drought