This happens to me all of the time. I feed 6 girls off the same res and at least one will be fussy. It looks like a Potassium and Magnesium deficiency or lock out? They look the same. If it is a lock out then the likely culprit is Cal-Mag but you are feeding a fairly low PPM and not all elements may be in balance. This could lead to an actual shortage in P-K-Mg. This is what I believe is the problem bump your PPM up to 600, add 1g Epsom Salt per gallon. Use the Astroflower in your mix.
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I'll up the strength. Doubling immediately won't be too much of an increase?
Ive also noticed small flies lately which apparently are fungus gnats ? There doesn't seem to be a lot, but I defoliated around the bottom for better airflow and ordered some sticky yellow traps and gnat nix which hopefully will arrive soon. From what I read, they can cause deficiencies, though I've had this problem basically every grow and this is the first time I've seen them. Im thinking from contaminated coco? I bought it from a garden supply shop but Im thinking it was stored outside /near other contaminated products.