I'm going to assume your high air ration in the res, creating lots of bubbles under the suspended perfect white root mass? If so my Dwc did the same, perfect white above water level. Yet for you it seems as if any root mass that's under water gets brown. You didn't mention if the roots were slimy or not, I'm talking the darker color section. For me, even a Sq inch of root rot caused noticeable problems. What microbes are you using since your not sterile with chiller?
Step one control root rot with microbes, hydroguard to be specific, double dose until they are not slimey.
Step one also, lower ppms drastically. Your plants are not light green really they look heavy with nitrogen to me,very dark green. That combined with the root rot would cause other leaves to be brown and dying from lack of nutrients.
So root rot and lower your ppms. You won't have any issues, not one, if your ppm slowly drops over a few days. You would much rather have it lower 200 points VS raise 200. Slower growth VS problems is all it is. The happy medium will only come with a very larger res side.
Id also raise the water level a bit so the plants do not rely on the fog.
Ideal photosynthesis takes place at 76f btw, I'd also turn my room temp up.
Whats your lux reading, you can push them once you get these issues lined out.. I'm off to bed now.. Laters
