Roots are probably just stained from that caca coloured water. IMO ppm is way higher than it needs to be. Are you sterilising the res at all? And what does the plant look like, if you have a root problem plant will be a tell tail sign
Is any of the root mass slimy? Ppm sounds too high, are u using bennies to keep the roots clean or are you using a sterilising agent? If ur doing neither read about sterile dwc, a lot, I’ve used bleach, peroxide, ozone, the whack lol