shiverled up leaves

Hey guys, i have just started to notice this on the big fan leaves. Any idea as to what's wrong, thanks!

setup-
400W HPS, promix soil, foxfarm nutrients, lights are 18inch from canopy, temps have been 70-80F, humidity 30-40% (a little on the dry side)

One thing I worry about is pH since I have only been using tap water and haven't been adding any PH down (did a great grow without adjusting PH so I've been lazy with this one in that respect). Oh yeah, I'm growing feminized "ICE" from femaleseeds.

over watering maybe? my plants looked like this and i slowed down on the over watering and it went away,

How old are they? Do you have a fan for circulation? It does look almost like a burn, perhaps nute burn but it looks more like a burn caused by the heat of a light but you say you have them 18" away from you light. Is the circulation good in your room good? Peace.

yeah i would check your heat. cannabis plants don't take heat fluxuations too well, so that could be your culprit. also pH could definitely be an issue.

Thanks for the replys. I will plan on checking the pH and see if that helps. I don't think it's a temp issue as the hottest it gets is like 80. Circulation is good, I've got 2 blower-type fans running when lights are on. The plants are 2 months old, second week into flowering. Being that they are femenized I hope I get mostly ladies and no hermies. Peace.