the roots should always be WHITE... brown is bad... unless your nuts are a real strong brown-red and your ppm is 1500+ ..... like dog lover said, "some nuts cause this" but if your roots are brown and not strong.. rather mushy... you are in deep SHIT!!

add Hydrogen peroxide asap.... bring air stones in.. do whatever it takes.. change your water and flush your system...

Dead brown roots are a serious problem.... they (dead matter) severly depletes oxygen levels and make matters worse exponetially

good luck