I have currently 200 plants. They are 4 weeks in to veg. I just wanted to flush them before switching to 12/12.
180 of babies are in soil/perlite mix and the rest 20 is in coco/perlite mix.
The ones in soil are perfect in condition after flush however the ones in coco developed brown spots on leaves. Leaves are very crispy and easily come off when I pull them.
It does look like calcium and magnesium deficiency to me.
Did you feed with nutrients for the coco plants after the flush?
Peat based mixes usually have some form of lime to adjust the PH which often provides a source of calcium and magnesium that is harder to wash out than liquid nutrients. Im guessing this is why your soil based plants look ok.
it's most likely a deficiency if you flushed all the nutes out of the coco. just hit them with some nutrients and remove the leaves with necrosis on them.