Any clue what this is?

Is this potassium? I have one plant out of 12 doing this. The others look okay.


DWC, plenty of air, good RH and temps.

Calcium?

yep, get some cal/mag for it i would say, although im no expert when it comes to nutrient def's as o use soil and seaweed meal and solution fish boold n bone meal

Thanks for the reply. I just hit it with a dose of calmag. It's ben a few days since I did it, I don't see it on any new growth, it's pretty much on two leaves. Two largest. They seem to be getting a little worse though, just a little, so not sure if it's slowing or not. It's RDWC so it shouldn't take long.