Having a issue that I can't identify. Please Help!

So I have one strain that seems to be doing this every time about 3 weeks out from harvest. All of the fan leaves turn this color. Not sure what is going on, if it is just the strain or something environmental etc....

Growing in 70/30 coco/perlite, EC is 1.36 in a DTW top feed system. Just triple checked the PH staying right at 5.9.

Please help me!

what nutes? do you use r/o or distilled water?

I am back to tap water, had some issues with RO water, I have since ironed them out an will be running RO next run. I am running JR Peters Hydro FEed, along with that I add in some Dynagro Bloom for the extra phosphorus, and Protek.

Been running this line up for a while, seems to be this one strain is picky