If you've ruled out PH... Then I think light is most likely the culprit. Iv'e had plants be at the exact same height and seen burn while others thrived. But just from the overall shape of the damage it looks isolated to the tops and does not look bad around the edges of the plant.. If this is indeed where the light is the most intense. Hard to tell without seeing your setup. After downloading and using a Lux meter for my phone I recommend it for everyone who uses LEDs now...