Yellowing leaves during flower, nutrient issue?

It is hard to get a good picture of, but the older growth on my 45 day flowering plants is yellowing - not just at the tips, but up and down the leaf. I've tried adding CalMg, upping potassium, nothing is helping. Any ideas?

Potassium. Your probably locked out.
http://catnews.org/FREE%20Pot%20Books/Marijuana%20Garden%20Saver%20%5b2005%5d.pdf

Are you growing under LED? You have some heat stress as well.

naw 600 watt hid, temps are at 80 degrees but I could move the filter outside the tent and get another ~5 inches of height from the plants if that would help