Thanks for that rockwool link, Weedhound, very informative. I also think the rockwool retaining so much water led to the problem. It was suggested to me that the over-watering has led to root drowning, which is causing a nutrient deficiency, and that I should apply a foliage feed of sea teas and half strength fish fertilizer... not something I would want to do if I'm experiencing over-fertilization. So, I'm still confused.

I'm using the same 600w HPS for veg. The thermometer has a long sensor wire, which I have dangling so that it reads from the plant tops. The plants are about 3 feet below the light, good ventilation.

Given the temperatures and distance to the light, I highly doubt that the issue here is temperature-related. But I'm not sure what exactly is going on, so I'm still confused as to how to fix things.