I recently had pythium issue when transplanting from a small DWC cloner which hadn't been amended with pool shock. My current grow is at my friends place. I'm only there for one or two days a week. He missed dosing with chlorine. Brown tan roots to begin with, smelled like rotten carrots. Took about 2 weeks with 1-2ppm residual for them to become pristine white all over again.
I also used Polysorbate 20 in combination to do this, but just for the first 4 initial days. Results speak for themselves really. Practical knowledge beats theoretical all days of the week.