I never got an answer to my first question, at least not that I could see..
The reason I ask what your PPM of your water is before nutes - is because I have had this exact same problem and I'll tell you exactly what it is.
Your roots in the pic look just like mine did. Pull them out of the water and run your fingers over them. I bet you've got root rot. If you run your fingers over the roots and they feel slimy - there's your problem.
Personally my biggest issue was I was using well water that had a PPM of 650 out the tap. My well was also infested with "iron bacteria" which gives excessive amounts of iron and manganese in your water.
Get yourself a good RO system and only use clean RO water to start.
The next issue is NFT. NFT is very hard to control root rot. NFT naturally lets more light into the root zone which in turn will end up giving you root rot issues. You need to seal every single port in your channels, make sure there is ZERO light getting in the channels either from the ports or the covers or whatever.
Next you need to make sure your rez. temps are stable. You can achieve this either by buying a cooler (expensive) - moving the reservoirs outside of the grow room, or make your own cooler. I made mine with an old radiator from a busted down Chevy Cavalier. You can even use the fan from the radiator or place a different fan in front of it, place it in front of an A/C unit, swamp cooler, whatever you can manage..
Lastly I can tell you 700 ppm for an NFT system is way too much.. After I finally got my NFT squared away I was running 250-350 ppm MAX, any more than that, I got all kinds of issues. NFT is great for picking up nutrients and quite frankly the best hydroponics system (and hardest to do) out of all hydro systems (in my humble opinion).
But I'm willing to bet money on
A) too much iron in your base water -
B) Too much light getting into channels or reservoir or both -
C) Water temps too high. - and
D) PPM of nutes a little too high for the type of hydro you are using.
Also, if you still cannot get root rot under control after all these measures - I recommend switching to a sterile system. This can be done very easily by adding a couple ml/gal of bleach to the reservoir every other day or so.
Let me know if any of these issues may be pertinent to your situation - as I believe they are. NFT is something of a pet hobby of mine
Edit to add - I just looked closer at your pics - are those plants in Rockwool cubes in the NFT channels?? That's a problem, as well. IF that's what I'm seeing - part of the issue is the rockwool is pulling water up, exposing it to light, and causing a breeding ground for pathogens. I had to custom design my own channels and to solve the issue of wicking water up, I used net pots with hydroton (clay pebbles) and made sure they were suspended well above the water line. You don't want wicking water up to the base of your plants in NFT- that will definitely cause root rot issues.