What is this

Hi, First of all it looks like your plants has enough nitrogen. The leaves are nice and dark green. The issue could be related to multiple things. To proper diagnose the issue we will need more information: 1. PH of nutrient solution 2. EC of nutrient solution 3. Humidity 4. Temperature 5. Watering protocol If i would guess just based on the pictures I think this is related to PH fluctuations . Cheers!

Over feeding.

Lookslike very slight nutrient burn. Maybe give slightly less when you feed next

Normally damage to the serated tips is K-related, but if not mistaken there;s a little blue hue to that 2nd picture? If so it's a phosphorous deficiency.