Looks like the start of nutrient burn to me, based on the leaf curling and the dried perimeter. It looks like a Mag deficiency, but I usually expect to see more necrosis on the middle of the leaves and not the burnt appearance of the leaves. Too much heat could also cause the effect, but without the middle discoloration. The plants looked pretty good up until the newest growth, which can sometimes show increased salt/nutrient buildupI use
Promix and use fairly low amounts of nutrients compared to many growers, but the plants look like they ate something that doesn't agree with them.