To me, it looks like the beginnings of a zinc deficiency. I had leaves that looked like that during my grow that went into the yellowing that were the tell tale signs.
Also, what is your humidity?
I just got snuck back up on with a zinc dif and had to go back with a couple real heavy waters with some epsom salt (light on the epsom).
Just throwing it out there..Again, I had leaves that looked exactly like that when I had too much sodium build up in my soil (because i used *drum roll*, water at the tap with perfect PH; right after it came through a water softener