Root Rot or ...?

Hello guys i have been fighting last 10 days with brown stuff on my roots and looking for conformation i am dealing with root rot.
I am using 2 single 25l buckets with 3 airstones in each and having these brown stuff all over my roots.
Was treating it with h202 12% every second day but brown just wont go away.
Keeping my equiptment sterile and water tems never go more than 21C.
One plant looks perfectly fine while other is fighting with this brown stuff.
here are some pictures of "sick" plant 3 weeks old. Pictures are taken after shower and soaking 10min in h2o2 solutin.
This is my first DWC Grow and its driving me crazy how much babysitting and work it requires.
Any advice would be very helpful.

Are your roots slimey to the touch? I run Dwc and do get slight browning on the roots but this is just the nutrients dieing the roots I believe.

Doesn't look like root rot to me... Are they slimey, smelly? Fall apart easily?

I would go w hydrogaurd instead of h202. I've tried both and in dwc hydrogaurd took the cake, it's a pitb to add h202 back every other day to the res

I totally agree hydroguard is awesome

The roots are not smelly just i wondering how in same setup with same nutes one plants roots are brown and others are clear white.
Unfortunatelly i live in ireland and need to order hydroguard online.
Is it sterille or have benneficials becose i use tap water so i suppose it has some chlorine inside. Reading is 50ppm from tap.

btw thanks for replay guys