with the KH and GH shouldn't he have enough Ca and Mg? How are you dosing No3 exactly? 2 tbls to 500 ml dry powder, etc? I would add more root fert, since you are using eco. If you had flourite, I wonder if you would be having the same problem. With that much light, your iron, No3 and phos have to be right on. I've had the same types of problems, but its been going away with lots of no3.