Well, there's a very good chance that in my case the problem is heat. My plants are in my attic, and even though I have a vent fan going, it sometimes gets 90 degrees outside, and it'll only be hotter inside. But I figure, if they can grow great weed in countries like Thailand, a little heat isn't going to do too much damage.

I know plants turn lighter green under HPS, I thought maybe the leaves were turning lighter unevenly. I have my dark cycle timed to occur during the hot part of the day, so the light actually isn't contributing much to the heat problem, it's just a matter of not having AC.

I found a picture of a plant with a slight magnesium deficiency, but it doesn't look like mine. I attached it, if anyone's interested.