according to the web
By lowering the temperature, you are allowing more oxygen to be held in the water, which your plants need to properly combat this parasite. Increased oxygen intake also has the added bonus of speeding up plant growth in general. Pythium comes when the water temperature is too high (usually above 72 degrees Fahrenheit). If the temperature is kept 72 degrees or lower, you will greatly reduce the chances of root rot occurring. If you have a good supplement, however, higher temperatures will be okay.
There are some white areas in that root system so there is still hope to save the plant.
You may find this helpful
Some people recommend treating cannabis root rot with H2O2, also known as hydrogen peroxide . While adding hydrogen peroxide to your water will kill most bacteria and pathogens, including the ones that cause root rot, it is only effective in your system for a day or so or since the H2O2 is quickly converted to oxygen and water.
Therefore, if you use hydrogen peroxide as a treatment for root rot, you will need to treat your water daily to prevent re-occurrences until you actually fix the problem that is causing your root rot in the first place. I have used commercial grade H2O2 against root rot more than once, and I never saw any noticeable difference, so I would NOT RECOMMEND H202 as any type of long term cure!
Why Not Use Hydrogen Peroxide? H2O2 is only temporarily effective at best when it comes to getting rid of root rot. It makes me so sad when people tell growers to use H2O2 (hydrogen peroxide) to kill root rot because I know they will continue to struggle with it. Even the commercial grade stuff just doesn’t work most of the time (and I’ve tried)!
All H2O2 does is cause bubbling near the dead brown roots (or any organic matter), which is why I think people believe it’s helping. In my experience, it doesn’t help the plants themselves (they don’t look any better), and what’s worse, all the H2O2 will be completely gone from the water within 24 hours.
H2O2 kills most of the bacteria in the reservoir, including any good bacteria you’ve added, and does nothing to address the underlying problem. You can’t sterilize away root rot, it attacks people everywhere! Although H2O2 does kill bad organisms, it leaves enough of it there to repopulate your reservoir.
I personally recommend Botanicare Hydroguard (the newer, improved version of Botanicare’s popular “Aquashield” root supplement) for marijuana root problems because “this guy” uses it successfully to get rid of more than once case of Root Rot (and by ‘successful’ He mean new white roots exploded out of the old mushy brown ones and I was able to eventually harvest the buds)
this was the short version I encourage you to read the full article. A lot of times this is attributed to hot water temps. Do you know what the temps of your reservoir were? I’ve read on other forums folks having success getting rid of root rot, but it may be best just to scrap it and start fresh.