Just to add, its definitely not an iron deficiency. The most common cause of iron deficiency is poor drainage and waterlogged roots. (EHG Micro has plenty of iron, that's where the brown colour comes from.) Because iron is not a mobile nutrient in the plant you'll notice it first in the new growth and not the leaf tips. Also, I am assuming your coco was well washed before you used it? In my experience MOST coco is very salty regardless of what the manufacturers say and that is particularly true of the 5kg blocks. Your runoff ec will give a clue as to whether that's the problem. Paul