Nope Iron (Fe) or Ir as I like to say here, makes the leaves look more white with a block or def.
I don't know what, or how much, or how often you feed.....that's important info you left out!
Now then, with that said. What it looks like to me. Is that your roots are getting too cold ant night!
Perfect picture to use to show that problem!
SO, how cold do they get? How warm during the lights on? Don't get me wrong , it still could be feeding related. So answer those questions too!