I can not understand that with plants different tips come most people think that the problem is nitrogen! Others say it's zinc or cal-mag! I can't decide how to properly help the plant! I also thought maybe it's too much blowing inside the box!?
Humidity is way too low, you need to be around 65% at this stage of growth. At 20% your Vapor Pressure Deficit is way too high which is stressing the plants out. Correct your humidity level and you should see improvement. Happy growing.