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potassium deficiency that is caused by ph lockout. Credit to: http://www.420magazine.com/forums/problems-pests-disease-control/81275-cannabis-plant-pest-problem-solver-pictorial.html

look up Zinc deficiency and see if that looks like what you have. That would be my guess. hope this helps

brown on cotyledons is just the plant using up the stored energy there (that is what they are for.) they are using extra energy trying to reach light. would lower light to about 4in as posted by KushisSweet.

Magnesium deficiency would be my best guess

They look a little dry. i would say underwatering and/or heat problems.

Just looks like early Mn deficiency. Also leaves are very dark green, have heard of FFOF soil being a little hot on the N which will give dark green leaves and can also signal mild nute lock. next water try a flush with water amount equal to amount of soil. with FFOF you should not need many...

looks like a calcium deficiency to me. but you may want to neem oil them to get rid of any critters/fungus to be safe.

https://www.rollitup.org/general-marijuana-growing/420048-nute-burn-deficiency-guide-diagnosing.html#post5505301 hope this helps.