Hey man, I lurk and don't feel like talking much, lol. But I can't stand watching your crop go downhill like that.
I know this one is all but done but just for future sake; I have grown in coco for 10 years now.
It looks to me like you're letting it dry out. Rule 1:Never let coco dry out, it is not dirt.
Here's what generally happens:
When coco dries out the nutes it holds become concentrated and you get some nute burn.
Also it whacks out your ph, usually going acidic. Newb's reaction is to lower feeding, or flush with plain water which is worse!
Rule 2: don't give it water with less than .8 ec, it fucks up the cation exchange.
By now with ph being off and on, along with burn you get deficiencies. All this will reduce your yield and potency and leave your crop looking crispy
even though you did everything right.....except let it dry out between watering. Now if I'm wrong I apologize, but this very thing is what stumped me when I switched to coco after growing in soil for 20 years!! Took 2 crops that looked just like yours before I got it figured out.
Stick with it man, I hope you get it figured out cuz coco really does kick ass!!
PS, I don't flush, lol. EDIT: it is possible to over water in coco if the roots aren't well established and seedlings. There's a bit more to coco than this little post.