For the past 2 years, I have grown almost exclusively in coco using GH 3 part. Prior to that I have grown DWC and soil and have never seen this issue. In the last 2 years I have grown 15-20 plants through flower and everyone has seen this issue at some stage. I have had people on various forums (including this one) tell me that it wasnt an issue or point to things like temperature or humidity. Given that I have tried multiple strains, temps, humidities I can only come to one conclusion, a DEFICIENCY or excess of something.
To prove this I grew a DWC plant and a COCO cloned from the same plant, in the same tent, with the same nutrients. The DWC showed no signs of purple fan leaves and the coco turned purple.
I became fed up with not knowing when my most recent grow started to show signs of purpling. Below are some pics, would really appreciate some help. Thanks.