Day 36 Flower - Tips of leaves going Yellow / Rusty

Hey guys just a quick question.

I'm day 36 of flower.

I've noticed that the tips of my leaves are beginning to go yellow / rusty.

The majority of the plant's leaves are the same, that is, beginning to go yellow or rusty.

(Images below)

I do have a fan blowing on top of the canopy and suspect' this could be a mild wind burn.

I hope it's not related to nutrients.

I grow in coconut core, run to waste.

My return waste PPM on average around 900 to 1100.

What are your thoughts?

Thanks.