legalcanada hasn't posted in this thread for almost 3 months and there are plenty of people using MC in coco, including me.
Look at the 85 page thread on Mega Crop!
https://www.rollitup.org/t/greenleaf-nutrients-and-mega-crop-happy-to-be-on-riu.946071/page-85#post-14410974
He did not say it's cal, mag, or iron!
He did say "Probably related to the balance between Iron, Mag and Calcium, not necessrily a lack of mag...could be pH, could be salt buildup, could be, umm, excessive Boron maybe?"
He's right about reading the label, MC is well known to have good amounts of Cal and mag.
"6.5% Total Calcium (Ca)
Contains 3 different sources of soluble Calcium including Calcium Amino Acid Chelate, ensuring maximum absorption in any condition. This, combined with a whopping 6.5% total content ensures there will be continuous adequate content for the entire life of the plant, even in heavy demanding stages. Calcium content is enough to meet even the most difficult growing mediums such as Coco Coir, which is typically known to leech Calcium from solutions due to the non-existent cation exchange capacity (CEC).
2% Total Magnesium (Mg)
Contains 2 different sources of soluble Magnesium including Magnesium Amino Acid Chelate, ensuring maximum absorption in any condition. Based on scientific analysis and plant tissue data, 2% Magnesium exceeds requirements for all crops, even difficult strains and genetics. Similar to our Calcium, the Magnesium content is enough to meet even the most difficult growing mediums such as Coco Coir, which have very low cation exchange capacity (CEC) and typically need supplements."