My plants are yellowing, what gives? (PICTURES)

Hello everyone, im having a bit of an issue.



at first i thought it was a Zn deficiency but now im stumped. Im hoping someone might shed some insight on the problem.
My older plants seem to be yellowing all over and the leaves vein areas are turning necrotic, starting at the leaf-node. My younger plants (about 4 weeks younger) are all fine and green (aside from some slight nute burn).
I would say around 70% of my modest crop has turned and its starting to get serious.
Ive been using Technafloras "recipe for success", which is bullshit by the way.

Nutrient info -> http://www.technaflora.com/products.php?product=11
Using tap water PH'd to 6.8-7.
80% Fox Farms Ocean Soil / 20% perlite in 6" pots.
1000w enclosed HPS, 18-20 inches from top of plants.

they started to develop yellow leaves near the top so my first guess was N deficiency, then Zn, but BC Grow contains N and Zn and it didnt help.
I then tried Schultz Liquid Plant Food 10-15-10 http://www.schultz.com/NR/exeres/537...8181668714.htm which contains twice as much Zn but still no luck. Ive tried no nutes for a week. Ive also tried putting nails in water and letting them rust for the iron. Ive switched to a 2 day watering cycle. still yellow, leaves still dieing.

Here is a young one beside an older one:


a closeup of an older lady:


Could they be outgrowing the 6" pots and thats why its only the older ones that are dieing?
I never intended for them to get this big in these pots but i had to put off flowering for a month or two.
heres some newer ones.


closeup of necrotic vein area:

transplant one,just pull it out of pot and look at the roots. they may be hard as a icecube.

Strange....I grow in 4" pots and never have this problem.....choose a couple of your healthiest plants and make alternating changes to one at a time, giving'em 2-3 days to show results.....take notes....something you are doing in mid growth is harming your plants as they hit a certian mark....my vote is overwatering which is burning your plants...typical of FFOF....that stuff is way over rated.....and misused most of the time......

Thanks guys, those are good suggestions.
I was in fact overwatering but have since layed off a bit with the water.
i was hoping the yellowing was reversible but it seems they are going to have to come off. I dont necessarilly want to clone them in this state but maybe starting over would be the best choice. *Sniffle*

So does anyone have any ideas of what i can do to salvage my babies?
Ive stopped watering so often, but the green isnt coming back.
Is there any hope or should i just salvage them by cloning and starting over?
Time is really no concern.

here's what I would do... flush all the yellow plants to get rid of anything in the pots, use one guinee pig plant and try upping the nitrogen 50% at least at first, just try it and see. By upping the nitrogen I mean using more of the high nitrogen food, easy for me due to using gh 3part (micro, grow, bloom) micro having the most N, do your best with what you got. Also just out of curiousity how much fluxuation in temps do you have from lit and dark? I've seen in some cases this can cause yellowing. Keep your PH at the right level, strictly. Also have a good look for bugs, they can be unseen but cause shit, but I bet you woulda noticed them by now, just sayin. I am no means a veteran grower but I have had alot of issues that I always seem to get to the bottom of. GL man,

~ BCbuddy