Need help identifying an issue please...

[Serapium]

Ok, I've tried numerous things in attempting to remedy this issue. I'm seeing this leaf development across several systems - organic living soil, DWC, and a top feed coco system. The living soil is getting mostly just water with a splash of calmag since I'm using RO plus a weekly dose of compost tea. The hydro systems are using GH nutes. I've checked and double checked my pH meters (three of them) for accuracy. The only parameter I can think of that I am struggling to get under control is heat. During lights on I regularly run at 85-90 degrees. No CO2 supplementation. Humidity at a solid 50% in flower. What does this look like to you?

[Serapium]

Here's what it looks like on an otherwise very healthy looking plant.

[az2000]

I'd say: K def?

[Mr.furley]

Agreed it looks like "K" is it on the older leaves or newer?

[Serapium]

Mostly on older leaves.