I have the same thing going on right now in my DWC using fox farm nutes. I was told it was a mag deficiency so I added some epsom salts but doesn't appear to be getting much better. The new growth looks great but once it grows out more it starts showing the same signs as the lower leaves slowly...
Just the very tip of the leaf curls down, yellows a bit in small spots starting with the edges typically then that yellowing begins to dry up and it appears to be working its way up so I'm guessing its a nute related issue.
I'm chalking mine up to big ph swings.. my shitty hanna meter crapped out on me so guestimating the PH a bit I'm sure caused the damage in my case. Check and recheck your PH and keep it stable for a couple days before you try anything nute related to make sure it's not just the PH locking things out.
