I just went through a case of some form of pythium which I'm pretty sure started in my cloner and hit all the phenos I had going, took a while for it to show up and the roots never had any signs (roots were white/clean/healthy looking). I tried some SM-90 which if anything made them worse, then switched to H2O2, gone.
Took about 2 months to save the last pheno but all of them are healthy/good to go again. While this may not apply or be as good for hydro systems, it worked like a charm in my 50% Pro-Mix/50% Triple Mix medium. The worst plant was an indica called Blue Pyramid that was so badly infected I was about to toss it, decided to try this and I'm about to flip it to 12/12 it recovered so well.
Cheap, effective, using it preventatively now through veg as it seems to help them/speed them up if anything:
35% food grade H2O2
Cut it down to 3% with filtered water
Add the 3% solution to my feedings
@150 ppm