Hey! This looks like leaf septoria (here's more info about it: https://www.royalqueenseeds.com/blog-leaf-septoria-yellow-leaf-spot-on-cannabis-plants-n678)
I'd recommend removing the infected leaves and controlling humidity (<40%) 🙌🏼
@James, well, I followed septoria guidance as well as possible. That is to say I removed all the potentially affected leaves on Northern Light, though Purple Lemonade would have been left with no foliage if I had done so for her, so I just removed the worst looking leaves. Still, gave the whole tent a thorough dry-out period. Northern Light has been pretty happy, and Purple Lemonade never started dropping foliage, so maybe she’s just a much hungrier girl than I gave her credit for and this was a CalMag thing. Or, as my partner has suspected from the start, just a bit of a sickly seed. *shrug*