could be a lot of things - pics will tell...
Could be MG deficiency - if leaf is curling up... epson salt will clear it up...
could be radiant heat burn from light hot spots... move lights up higher...
most likely - Nute burn... VERY VERY easy to occur in hydro (DWC especially).... FLush with PH'd water (no nutes) for 3-5 days, and try less nutes after...