Marrijuana leaf edges folding under

Title is self explanitory. All of my plants have 30% of leaf edges folding under. 600w hps is 22-24 inches away. so i dont think its the light. Seconf wk of flowering. Any comments or suggestions are muh appreciated.
FOLDING EDGES ARE CIRCLED IN RED

My plants have the same thing. What's your ph? Soil? Water?

Heat burn usually causes cupping leaf margins...upward and twisting. More info is needed...

temp is 74 deg F. and humidity fluctuates between 40% - 60% and the hps light is 22-24 inches away from tops of plants.