Chow_13answered grow question 3 years ago It could be just root stain.GH nuturents can do that. But they do look rotten.
Thoroughly clean your hands and feel the roots. Are they slimey? If yes then spray them with 3% hydrogen peroxide. And do a water change adding 3ml/l hydrogen peroxide. Repeat 2-3 days later. Hydrogen peroxide will kill hydro guard so start using it after you clear up the root rot. My guess is light leaks from your net pots is the cause. Or tos above 70f.
Good luck
Happy Growing