Who grows in coco?

You definitely need dry spells between watering. Natural soil is 25-50% gasses (oxygen, nitrogen), that's a lot of holes for the roots to actually breath. That's why you need the oxygen % high on hydro. If your substrate is full with water your roots won't breath and will die. Dead roots get rotten, alive roots not, that's how you get "root rot". Once they're recovered, you can start a feeding schedule to take care or at least mitigate the yellowing. Keep on the great and close care! Happy smokes!