Unknown Issue, deficiency?

Have five plants, all planted at the same time and are about 2 weeks old. Noticed some graying on the lower to middle leaves on one plant (gigabud). Few days later, the leaves are gray with reddish/orange where the gray meets the green, and the leaves are so frail when you touch it they crack off. The other 4 plants are just fine. They were given 5.8 pHd water with no nutrients up until last week (growing in coco, using calmag, h&g cocos a and b with root excel gold, using half the recommended amount). They are under a 300w LED with temps around 77 and humidity 60%. So far I haven't been able to identify/match this to any deficiency I've seen.

May be related or unrelated, but after I planted, I noticed that the bag of coco I was using contained hypoaspis mites, which I've read are virtually harmless to the plants, and I have been treating the coco with neem oil once a week. I figure if they were eating the roots, then all 5 plants would have similar symptoms.
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