So raise the light. Problem solved. 
seriously though, I have two Zak Haze clones. Same plant, same soil, same tent, everything the same except the yellowing one is under the COB and the greener one is off to the side more.
Too much light is a real thing. I think most HID users dont see this because they cant get the light close enough without the heat (IR) becoming an issue. Kind of like a self regulating system.
I was going to suggest the same thing after checking all the common possibilities. I’m an HID grower and I’ve experienced it with a 400 watt MH.
Yes, lights with high intensity at a certain distance can cause problems.
I do think the light is exaserbating whatever the issue is, but I should be able to hit around 50k lux without issues as far as I have read on other sites using COBs and strips. Have you ever taken any lux readings per chance? I’d be interested to know what they are at canopy if you happen to know.
I do agree though, I’ve seen many posts that turned out to be light stress with cobs/strips.
Nah, the meter on my phone was far from accurate. A nice PAR meter is on my wish list though.
The only thing I would suggest is to bump up the nutrient ppm up to 550, that would be 700 with tap.
@ 350 the plants could be running out of some Micro and macronutrients nutrients causing possible deficiencies.
This. I do the same. I tried poolshock but only had issues. Either leaf burn or roots never wanting to get into the water. Switched to bleach and haven’t looked back, no issues with it.
Seriously? I use it at 0.1/0.2ml per gallon.
Yup. Plants didn’t seem to mind and it’s cheap as chips.
Should be about 15 ppm with the 6% stuff. It dissipates rapidly, and I wasn’t too good about redosing every couple days.
That does seem pretty high man, but ifnyour plants didn’t mind that has me wondering…
I was never putting more than like a 5th of a teaspoon of pool shock into my 8 gallon rez 
ah gotcha, mine is the 8%. I think i’m running around 3-4ppm. Read more than 6ppm would cause damage in herb but never tried. Good to know might be safe then.
How much pool shock were you using when you had issues? Not sure if you measured or not
I was still trying to maintain the same 3-4ppm while using poolshock. Think I was doing like 0.2g/4gallons, should come out to around 5ppm. But yeah roots wanted to stay out of the water and nothing but issues.
Ok so one avenue I’m exploring might actually be making some sense…
I was wondering if frequent irrigations (every 2 hours) might make a difference in nutrient uptake and indeed they do…
I looks like NH4 will basically increase in the medium as the time between irrigations is reduced. This could lead to N tox which would explain the dark green leaves and clawing even at low EC:
https://www.ishs.org/ishs-article/627_10
Time-dependent processes such as nitrification reduce the temporal NH4 concentrations in the rhizosphere and hence, the actual NH4 concentrations increase as the time interval between consecutive fertigations is reduced. Adjustment of the NH4/NO3 ratio at high irrigation frequencies is recommended, in order to diminish the risks of NH4 toxicity in sensitive crops.
Also it looks like if fertigating frequently I can reduce the amount of immobile elements such as P and K, which I am not doing… which could lead to an excess… And some pictures of K tox I have seen do look like what I have going on.
https://www.jstor.org/stable/24121202?seq=1
Nothing conclusive here, but it does suggest that if I got back to 6 inch blocks and water only once a day, I’m less likely to see nutrient issues at lower ECs…
I was under the impression you can go up into the low 20’s. Hell, most tapwater is 5 ppm to start with.
It should be noted I only added it during a reservoir change, which was every 7-14 days, and it dissipates very rapidly. A lot of guys add it every 3 days which may be a bit overkill.
well that makes more sense
You COULD use it to adjust pH, but those levels wouldn’t grow any plants.
It would get your laundry whiter than white though haha.
Or clean the dirt from under my fingernails. Love that trick.
Sorry to bump an old thread, but this same exact thing has been happening to me for a couple grows now, and it is driving me crazy as I have tried everything. No matter what I do the leaf tips just keep coming out yellow. Last time this stopped when I switched to flower/bloom nutes, but on my current grow it is now sticking around despite flipping.
Wondering if the OP has solved this or anyone else has experienced this? I have a par meter and the light is a good distance away, I even bought a new blue lab PH pen. I thought it was a mag deficiency but adding more didn’t help. I am growing in coco.
Last post OP made discussed the possibility of high N, especially when feeding frequently, and I do high frequency fertigation (5-6x per day). Wondering if high N was the issue causing lockout?
I am at a loss and it is damn near depressing 🫤
I would begin my monitoring PH run off. I was having this problem in the past. Once I began monitoring ph run off and steering input based on those numbers I’ve been having much better runs. My run off numbers were getting way off around week 5 / 6. I hypothesize it has something to do with the medium being taken up by roots and not having the same buffering qualities.
Thanks Hoss, I’m mixing a new batch of nutes as we speak and I am going to check the run off PH.
Post some pics if possible … 
Hey man, (op here) I did solve this issue and can only give some theory as to how… I can nuke my plants with light now, 70k+ lux at canopy without any weird leaf coloring happening up top.
I’ll try to note all the things I changed.
My guess is this:
My best grow (yield for space and quality) probably ever was in rockwool, but I used 6 inch cubes and I really let plants get bigger before flipping to flower. I was also doing flood and drain back then with a single flood every day. Plants were blasted, I literally measured 90k lux on top of a Dinafem Orange Juice and no weird leaf colors in sight.
When I switched to frequent irrigations and smaller rockwool blocks, and smaller plants (more plants per tent) rockwool didn’t seem great for that. That’s when I had the problems in this post.
Coco allows me to run tons of small plants in small containers and is really flexible on amount of nutrients AND on how frequent you water.
IMO Coco is where it’s at. The only advantage Rockwool has IMO is the cleanliness. 0 mess with rockwool and I do miss that… BUT for any growing where I actually care about the end result it’s Coco 100% for me.
