Please help:)

Hello guys, I need some help I am growing 6 plants under 2 400 watt hps and only on of them is curling leaves. I dont know whats wrong, direct sunlight temperature is not more than 40° and humidity seems okay too, grow room temperature is about 30° and air circulation is quite good. I dont have any grow box its just a big room(2,5x2,5x2) where I let fresh air only when I visit them twice or once a day. I was watering all plants with tap water, but yesterday I have bought ph tester and adjusted ph level of water to 6,5. What else can cause this problems, because other 5 plants seem healthy and are growing quite fast. The strain is purple haze, I will attach some pictures to help you understand whats wrong. Thanks

This picture was taken 2 days earlier

The pot looks pretty dry, Amigo. It causes the leaves to curl up like that. If it was a nutrient issue, you'd have spots and rust colors.

You need to be more diligent with your watering routine. If the water runs out the bottom shortly after pouring, that means there's channeling occurring in the soil, keeping it from soaking in the other areas of root mass.(it takes a little handiwork to right this) I know for a fact you're under watering because in both photos (which you say are two days apart) show a bone dry soil surface...that should not happen. Watering should also be thorough...in other words, a lot of water less often is better than a little water all the time.

heat stress , did you move indoor growing plant outdoor under sun ? high temp or light too close to plant ( for HID or LED lights )

That's definitely heat stress from your lights. I'd only run one 400w until they get a little bigger

Or raise your lights