DWC GROW (first time ever) - GH - IRAN

Yellow leaves are usually down to the Ph of the water, ph may be too high or too low so you should check the Ph of your water to see where it is at. Ph 6.5 is good. Do the leaves feel swollen or papery thin? If there swollen you may be watering them too much or if there thin then that is a sign of not watering enough. What is your watering schedule like? EDIT - just noticed your using hydroponic system, I would check Ph of water 💦