do you have holes in the bottom of the cups to let water drain out?
one of the problems with soils like MG is if you over water them you wind up releasing more of the nutes. this causes two bad things... over nuting at first and then following up with under nuting as time pases because you have washed the nutes away.
the problem sounds like a typical nitrogen starvation and over watering.
if you transplant them into a larger pot with more MG soil and stop over watering them they should improve.
no, i am not advocating using MG soil, but it does work... sorta
Little Tommy sure aint wrong.
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wait, let me get this straight... if you fry an egg you can get fucked up?

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