I believe it is the Trichoderma in the bennies which fight of pythium.
Completely got rid of my root rot the very first time I did hydro. Fought it for a week with H2O2, then threw in the Bennies and problem solved! Not just solved, but the roots LOVED it!
I'm at 68F (20C) with bennies, seems to be their happy place.
They look good now. Just know if the causation of the event was not fixed, the root rot will return.
You need regular H2O2 applications, or regular Tea/Bennies Feedings. Or fix your water temperature.