Is this light burn or something else? Maybe potassium deficiency?

Hello,

This is my first grow, I'm on the first week of flower after about 9 weeks in veg. I suspect this is light burn based on the well-defined line of damage in the first and last images. I also lowered the light slightly just after flipping the light cycle. Absolutely no problems preceding this, the plant is (seemingly) healthy otherwise. I've been adding under 1ml of calmag to my water along with the nutrients, because it's a soil grow I realised I don't need to do this. Could this be causing a potassium "lockout"?

PICS HERE:
* TS600 light
* Tatanka Pure CBD
* Soil/perlite
* GH Trio nutrients - (currently 2ml Micro, 1ml Gro, 2ml Bloom)
* Watering approximately every three days, or when the pot feels light @ 6.5-7ph. Intermittent plain pH'd watering.
* 15L Air pot

Thanks!

Can you post some pics of both old and new growth?

has it just started? or has it been a gradual thing. i can see some nute burn,

go go kid said:

has it just started? or has it been a gradual thing. i can see some nute burn,

Click to expand...

Yeah it started overnight. I just noticed some more damaged leaves now.

GBAUTO said:

Can you post some pics of both old and new growth?

Click to expand...

Sure, I noticed today another older leaf had the rust damage (fourth pic). The colour saturation on my camera seems to be a little weird, the new growth isn't actually as yellow as it seems in some of the images.

Pics:

My last grow done this. I added a little extra of my normal feed and they sorted themselves out. I presumed it was potassium but maybe nitrogen for the stretch too

I noticed another mildly affected leaf today. Can't figure out what's causing it.