How is your PH? PH fluctuations can be an indeicator that you have molds in your system. So can PPM fluctuations.
If you have root rot you need not only to cure it but also to find out why it happened in the first place.
Can you post more details of your DWC system.
Important factors are.
Water temp - Warm water can not hold as much Dissoloved Oxigen as cool water. for optimum growth you need to keep it at 68F (3 -/+)
Airation. You need to be keeping that liquid airated. I use 2x Airpumps per 20Liter Rez. Thats 450Liters Per hour, into three 6" air stones. (Next grow I will add more.)
It is also good to keep the water moving, A power head or submersable pump does this job, they also add more air to the mix.
Rez change. How offten do you change your rez?
As my Rez is small (20Liter) I change it once a week. when I do this, I scrub everything clean, make sure all the pumps are working before putting them back.
Water level, depending on the size of your rez you will need to top up once or twice a day, to keep the water level right.
The most likly course of Root Rot is a lack of oxigen in the water, most commenly this happens beacuse the water temp is too high. You can have hundreds of bubbles in your rez, but if the ogegen is unable to disolve into the water, you will have problems. Altatued also effects this, so if you are a few 100 feet above sea level, the effects will be more extream.
As for a cure.
First find out what has coursed your problem.
Clean everything.
Flush the system, with Plain water and H202, for a day or so. Then change out the water, and use an Enzyme to eat dead root matter.
If you have any root stimulators these will help.
Use low nute loads, as your roots are damaged and won't be able to handle high PPms.
Change out the water frequently.
Befor you do this make sure it is root rot. Post us some pictures of your roots.
If you have rot Your Rez should smell something like rotten cabage.
You could just have stained roots from your nutes, so find that out if it is normal first.