What is this?

http://imgur.com/UtAge9a

a few of my big lower fan leaves are starting to do whats in the picture. The rest of the plant is doing great! I'm about 40 days into flower. Is this a deficiency?

Looks like phosphorus deficiency. Using bloom nutes? The plant uses a lot of phosphorus in flower.

Tell us about your set up.... soil? Are you giving nutes if so what kinds? How is your ph? Is it only leafs by a fan? Sometimes wind burn can make a leaf look like that. Honestly more information is needed

3 gallon smart pot, 100% coco, auto watering system feeding 3 times during lights on getting a bit of run off, using heads 6/9 + cal mag

Flushed it with some ph'd RO water plus some foxfarm sledge hammer and it seemed to stop.