I'm not used to hydroponic, but I think
@The8thChevron must be right here, hydrogen peroxide (H₂O₂) is my best root saver : it will "eat" the rotten roots, cleaning them of fungus and bacteria, liberating molecular oxygen in the process.
Just be careful to use only "Food Grade" 3% Hydrogen Peroxide solution, because non food grade have some stabilizers added into it. Usually safe mix will be 3ml/L of 3% Hydrogen Peroxide with clean water.
By the way : use gloves and protective goggles.