Hello all i have same leaves on this plant that are spotted, mostly not older leaves but more like between the new and old ones, looks like cal def? But i shouldn't have problems with my coco ph as its around 6.. and i have high ppm will tackle with next watering
Cold temperatures then ,something is causing it. 15.8 way too low, at 15 cut off zone of activity for weed, raise a bit. It lock outs calcium too ...
It could be damn sensitive genetics as well ...
Temperature never passed 25.5c and at lights out it goes down to even 15.8c, slight breeze really not to much just leaves looks like they are vibrating so i guess i have good circulation too
When they get cold issue, they look droopy generally, yours like praying, so I would check temps at top canopy level. Don't go further than 27 cel. Target 25.
I gave my guess based on your words and believing that hat you do everything good with food and pH, then only too close light, or too low temps can mix calcium
Lights too close and intense can cause it too. Looks like deficiency, but in reality burn out and incapability to photosynthesis because of calcium issues cause this.
All the Best !