i find that happens to mine only under hps light, leaves get a light green kind of yellowish color, otherwise completely healthy. Either add a blue spectrum, or leave them.
If not, your soil might not be breathing, if your soil is too compact, otherwise maybe a nitrogen def, or root bound
No, nitrogen deficency starts at the bottom, sulfer deficency causes
the new growth to go yellow.
Could also be light bleaching or root bound.
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