leaf curl, help please

My buddy has his plants in the 24/0 veg stage with 600 watt HPS lights.
Temp:80
Nutritional good, water PH balanced to 6.0 he also has an oscilating fan blowin between the lights and very top of the canopy.

His leaves are beginning to curl up long ways like a hot dog buns or 'praying for magnesium' they're not turning yellow, and its happening to plants with lighter and darker leaves alike. He added alittle epsom salt to his last watering 2 days ago, and still no response.

Draft? lack of CO2? lack of fresh air? What causes this? He would VERY much appreciate some help here.

using a water meter they're watered at 1-2 on the 1-10 moisture meter. (Dry is 1-3)
He was told to let the soil completely dry before watering, just not to let it stay that long.

if your using soil as your growing medium, don't you think you should have your ph closer to 6.5? but besides that, maybe its a drastic change in temperature from light to dark.

There is no dark, its vegging. really? nobody can help him lol?