if your getting root rot then you have light getting into the air tube as light is getting at it, thing with dwc is you have to keep things clean including having a spare bucket that you switch out with the other so you can keep things clean, i would clean every thing and then i would run just plan water for a few days and you should leach out any of the nutrients that is causing the black build up, dwc buckets and air lines have to be cleaned, now things makes sense as i seen your other question. as far as root rot it looks like your roots are fine, if you have root rot you would know cause they smell real bad,
smell your roots to see