Is this happening on every leaf or just one? Can you post a pic of the plant? What is your feeding schedule and ppms and ph and temps? This info will be helpful for the hydro guys to diagnose i am sure.
Water temp in dwc is 79.2 °F and ph is at 5.8 ....I had a ph problem 1 week ago that's why the lower leaf is all cover with brown spots but it has been fixed.....in the dwc I change the water and nutrients once a week to the recommended amount per liter... I use the nutrients+ grow kit and follow there feed chart