How’d this old thing get bumped? 
The plant this came from is about to finish flowering. It had plenty of other problems after this 
Anyway, leaves were lower fan leaves – only old growth was impacted.
I did end up getting an RO filter in response to this thread, seems to get the job done. Have had a multitude of other problems since then, particularly in veg – my current suspicion is slow-but-steady overwatering, probably leading to soil pH issues and weak root system – I’ll update this thread with progress soon.