Hi :)
So here's my review of Pythoff!
Bit of background info - I have had Pythium in the past and it's an absolute nightmare when it sets in. I bought this on my first RDWC grow when I saw brown roots and it just... works!
I now use it at my reservoir change days at a rate of 20ml/200l as a preventative measure along with H2O2 - now, most people think I'm mad but you know what? I always get healthy plants without root issues with this method :)
It isn't cheap, but a bottle lasts AGES! Smells like swimming pool too :o
I cannot praise this product enough - it means even with my chiller, if there are issues with high temps I know I am covered - esp in conjunction with H2O2