Your ph at 6.1 is ok but you don't have the ppm amount on your page, it could be either nute burn or over watering but going by the pictures it doesn't look over watered so my best guess is nute burn but if you want to know for sure do what gmslave said.
Wanna be a plant doctor? Check your runoff ph and ec levels. That tells you what's going on. Low ph or ec, high ph or ec. Adjust whats out of whack and you just cured your patient. Cheers