Coco holds onto calcium and magnesium and hands the plant potassium instead, which is why CBD plants in it throw rusty spots on the mid leaves around week three of flower unless you add cal mag from the start. The problem looks like a fungal issue at first and gets misdiagnosed constantly. Dosage by growth stage, and why hard water areas can skip it entirely, are worked out on https://calmcbdseeds.com (https://calmcbdseeds.com) in a short coco specific guide. Buffering the coco before use prevents most of it.