What's Going On?

I just posted a moment ago about a different issue but I think you are dead on with the call of Cal-Mag deficiency. I started with similar issue to what you have and cleared it right up with CaliMagic. Good luck!

This may help too. It is what I'm using until I can get some advice.

https://www.thcfarmer.com/community/threads/there-is-no-such-thing-as-a-cal-mag-deficiency.62747/

I would move away from cal-mag and get some epsom salt and biomin calcium. The calcium is fully chelated. Most likely you have a lock out. To help this you need lots of RO or distilled. Run about 3 gallons of water and then on the last gallon run calcium and Epsom salt. 3 days later do it again except this time run a low dose of nutrients and add calcium again. The purple stem clear up from a proper dose of Epsom salt. I usually do 1/2 cup epsom to 30 gal water and I'll do 4-5 oz calcium to my res. Of course you can scale it down to your needs. I would do a tsp of Epsom and 2 tsp calcium. Clears up over a week. Check new growth for necrosis or spots

I would also stop using any pH up or down

Excuse the newbie question but is RO or distilled water already at a proper ph? Or why would you stop?

Leew421 said:

I would also stop using any pH up or down

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The soil is the buffer. If you add pH up and down you throw the soil out of whack. Ro or distilled is 7 and very neutral.

Leew421 said:

The soil is the buffer. If you add pH up and down you throw the soil out of whack. Ro or distilled is 7 and very neutral.

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While RO/distilled water should have a pH of 7, it has no minerals in the water so therefore it has no buffering capacity. This means adding pH down to RO water will reduce the pH much more than if you were to add pH down to hard water. The minerals in "hard" water act as a buffer. I would agree that the soil acts as a buffer, but I don't see how adjusting the pH of my water will throw off the pH of my soil. I tested the pH of my soil and it was 6.5, the same pH of the water I put in the soil. The only thing I can think of is that salts formed from adding acid (remember your standard neutralization reaction of an acid with a base) and this may inhibit absorption of nutrients at the roots, but this seems rather farfetched. It definitely seems better than the alternative which is to not add any pH adjuster and give the plants highly acidic (pH of ~6) water when adding fertilizer, which would definitely cause nutrient lockout.

The pH up and down stay in the soil causing issues that shouldn't be there. Just cause the water goes in a certain pH doesn't mean it will stay.

That doesn't appear to be ph fluctuation to me. It doesn't look like a common nutrient deficiency or toxicity. That could be to early stages of root rot. I hope I'm wrong!

I'm 29 days bloom today and I don't have nothing like that going on.