Deficiency or ?

First off, here's a pic. I scanned an affected leaf because the res is a little better than my phone (and no EXIF data...)

It's only on the older fan leaves and only near the bottom. Eventually they curl along their length, dry to crispy starting at the tips and die.
It looks like rust spots which is textbook calcium, but it's starting from the veins rather than the margins and from the base rather than the tip.
The only things that I've changed are the strain and the lighting and this is the first time I've seen this.

-6 weeks veg under 1000W MH, light about 30" above (as close as possible w/o burning tips). My last grow was flourescent.

-Organic "fruit & veggie" potting soil (same stuff I've always used from cradle to grave)

-No nutes, water allowed to sit.

-I added some neem (& tiny bit of diluted dishsoap as an emulsifier) to the last watering for fungus gnat larvae - I haven't grown in a while and I overwatered earlier. They're drying out now and this problem predated that.

This is from the oldest plant (staggered grow) but the others are starting to show it as well so it's something systemic like a deficiency or pests.

Thoughts ?

I would tend to say either P or K deficiency, but even though you say you're not feeding, that leaf liiks kinda dark, like an abundance of N.
What's your avatar saying? It looks like "Mia" from 2 1/2 men, and looks like she's saying 'Paul's got balls' LOL
View attachment 2444707

That one leaf might look dark but it's probably because I scanned it rather than taking a pic - the rest look pretty normal, sort of a medium green. Seems to be improving since I moved the light up and flushed 'em, so we'll see.

You know, I picked that for an av because, even after watching it for a while, I still couldn't figure it out,...so I watched it some more and still couldn't...