What are you growing in, Current feed schedule how much of what and how often. Whats the PH going in and runoff, whats your EC values going in and runoff. Light used and distance. Picture of the whole plant helps a lot. My guess would be nutrient burn or light bleaching.
Im growing in spil and using the whole line of advanced nutriënts and using 2x mars hydro sp150 highed is 50cm. ph is around 6.5 and ec i dont know, have not a ec meter