northern lights grow getting some yellow leaves on one plant!

whats up everyone, im trying to figure out whats going on with one of my plants. I have 2 NL plants flowering in a 2'x2' mega garden hydro flood and drain system. its in a closet that's about 4' by 8' and about 8' tall. I have 1300 watts of HPS on them lol 1000 watt at the top and 2 150 watt at the bottom. one of the plants has a bunch of the sun leaves that are yellow and some that have died.. ive done a lot of research on finding out hwats going on but I have one plant looking amazing and one that's not doing good that are getting the same nutrients and same light.. the one not doing so good was more centered under the light so could it mabe have gotten so much light it was starving for nitrogen or something? my temps are great, great air flow, kept my ph on constant monitoring and within 5.7-6.4 and from 800 ppm up to 1000 ppm fert and also just started dosing calcium and magnesium. any ideas? the roots are still growing really fast and healthy and the plants are about 3' tall. also the yellow leaves are only in the bottom half of the plant the top half is nice a green I just don't want it to creep up higher haha

anyone? it doesn't seem to be getting any higher in the leaves, I am having to use ph down about twice a day as it goes from 5.9 to 6.3 in about 10-12 hours very consistently ive been doing that for about 1 week or 2 now and water changes once a week

Without pics its hard to judge but could be heat stress from lights.

yea sorry I don't have a way to get pics on here lol the temp its about 72f 40% humidity ive seen burn before its not that, ive pretty much narrowed it down to the only thing it could be is starving, right now im trying about 900ppm of nova grow with about 200 ppm cal-mag ill post the results when I see something, hope im right! lol

I was typing something on another thread but it got posted here and I can't delete it. Stoner move

I don't know if this is just a craaazy coincidence, but a few years ago I grew 4 NL plants, all clones from same mother... just ONE of the four plants had yellow leaves the entire plant life after a few weeks of veg! Same DNA, same feeding schedule, but just the one was yellow and stayed. It did fully flower and produce, and actually out weighed one of her sisters! No loss in quality. This have NEVER happened since, with the NL strain or any other for that matter.
Other small notes, the yellow was very solid and consistent, besides the non-green color, everything else about the plant was fine and turned out great
Just thought Id share the story, feel free to ask any questions if I can help