Heyy dude!
I'm not the best in sickness, and never grow in DWC, but try to help,maybe some expert will come and correct me.
Yellowing sometimes tell you N def, but it can be K def I think.
Do you use a PH meter? Maybe PH problem.
Or... At once when I watering my plants some water drops fall on my plants leaft (nutrie water) and after that when the lights dry it left in the same spot like yours (if I remember well).