3.5 weeks into flower. This deficiency popped up pretty recently
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Time to call in the doctor… 
@George, does that look like the start of a phosphorous deficiency to you - or something else?
Cheers
G
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Dealing with a similar but not identical issue. I’m going to pull up a chair and listen in.
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What is your medium? What are you feeding? What is your PH? What type of water, ro or tap?
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Thanks, I don’t see damaged borders so I would go for Magnesium instead, they seem under LED lights so could be … 
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You are right, that’s a better fit. 
Cheers
G
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Does look more like magnesium. I am curious about the environment. I find many use tap water full of calcium and throw calmag in causing excess calcium which locks out magnesium. Than try to remedy it by adding more calmag instead of lowering calcium and adding Epsom salt. I could be off though.
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Half are under leds, the others are under a cmh
Soil, I’ve mixed up some coast of main soil with some ffof.
I’m using tap letting it sit for a while. PH of tap is about 7.5
I’ve been taking it down to 6.5-6.8
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I had just recently watered, I had a sample from geoflora, that I’ve been using
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my question is how long did it take to go from looking normal, to looking like that? We talking “overnight or took a week”? kind of thing.
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I watered Saturday and I didn’t see any deficiencies. Then this came up, so It could have been there before I watered. I just didn’t notice
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Sorry, couldn’t resist. 
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Magnesium and phosphorous are synergistic partners. Often one goes deficient as a result of the other. Could be either IMO.
If you are feeding using a recirculating method, keep in mind that N/P/K nutrients will deplete faster than Ca/Mg. So if you haven’t changed your reservoir in a while, I’d start there.
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I’m in soil, no reservoir here. I was thinking of using just a magnesium supplement .
I could test my water for calcium, I’m not sure if that would help. Just don’t have a magnesium test kit.
Thanks guys, hopefully I’ll get it straight
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Bump pk and calmag they want more than they got.
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Also more fluid once at least water em real good with some epsom salt if you just want mag not cal then once nice n moist throw pk booster on there. Dont overdo it just lil bump
I’ve had this problem with my Northern Lights and damn if I couldn’t figure it out. I tried more calmag, less calmag, more ferts, less ferts, pH adjustments and nothing worked.
The good news though is that the plants finished just fine. Many of the fan leaves were quite ugly and she wouldn’t win a beauty contest but it produced nonetheless.
Good luck and all the best.
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Too much magnesium
Decrease calcium
Add lime
Outdoor trick
Soul acidic this will only delay yield growth etc…