I've only ever had problems with extra humidity, not lack of! And it's dry as a mofo up here in wintertime. The air crackles with static. But maybe I've just been lucky, who knows? Normally I have to watch for damping off in seedling stage, and then mold in flowering. Maybe I tend to grow strains that like things dry, I don't know.
EDIT: How come some of the plants are bright yellow? Or is that the lighting? hahah.
That was due to the low humidity. What happens is when it gets to low, the stogmata will close, and the plant does not absorb the nutrients, and the soil will get acidic, causing a Cal/mag problem.
Makes sense that it looks like a magnesium deficiency. I would assume you had enough Mg in the soil or water, it's just that the plant couldn't uptake it properly because of low humidity. Is your house extra dry, or do you live in a dry region? I'm fairly dry here, but so far haven't had any problems like this. I just grow right out in the dry living room air and they're happy. Man there's so much still to learn.