Close up pictures next time please! To my it looks like damage caused by the leaves hitting the pebbles, most likely at night when the leaves are lowered and the fan is blowing on them. It is quite common and relatively "normal", so nothing really to worry about, as long as the new leaves are undamaged and the plants keep growing. Cheers, Organoman.

Just like my buddy said, regarding the pH, it could be, but... Normally, it's different from what appears on the leaves. If you want, open another question with closer photos of the leaves, please.

Hey buddy, how's it going? From the photos, it's not easy to see exactly what it could be, they're a bit far away, but... At the same time, it looks like Leaf Miners, a bug that eats the leaf from the inside. IF that's the case, it's easy, just remove those leaves. And don't let it spread to the others, but once you remove the leaves, the problem should be solved. The upper leaves don't seem to have issues. Good luck with the grow, hugs!

It looks like pH swing... I would definitely check the pH and adjust accordingly