I have 4 GG#4 in veg under a 400w MH in a 4x4 tent. Temps are in the 21c - 24c range and humidity 40-45. I have 1 fan on lowest setting blowing across the canopy. I run 18/6 but it seems after approx 15 hours of light the plants get droopy and the top new growth closes up together as seen in the attached pic. Overnight they return to normal and look perfect in the morning.
Drooping and some cupping. My quick guess is that you probably have the lights a bit too close to the canopy. Temps may be fine, but if the lights are too close, their intensity could cause the top leaves to cup, twist and even droop.How high above the canopy are your lights?
I've had a crop that did that with 16hrs lights on. When I went 12/12 they still did it before lights off like they was going to bed early. Really annoying! I pulled them all. They were Bagseed anyways.