Get rid of your organic Fox Farm and run a synthetic like GH. Any organic load you have in your water is going to be oxidized by peroxide or HOCL rendering it useless for protecting roots.
This. Using nutes with organic components is a bad idea in hydro. Bacteria/algae just love that crap and will grow like crazy. GH, or Jacks 321 hydro mix or the miracle grow stuff are all good pure salts and will reduce your bio problems to a large degree. As an added bonus, your PH control will be easier as any bio activity drives the PH up.
Speaking of PH, your keeping it too low. You should run 5.5-6.1 with 5.8 the ideal average. Its good for the plant to have a range of PH, so dont get too hung up on keeping it in a narrow range.
I suppose its possible you may be getting clumps of undissolved nutes on the roots, but I really doubt that. You would have to be dumping in a ton. It would have been easier to tell if you had a pic.
But - the pic you show is NOT nutes clinging to the roots or staining them. That brownish crap is the beginnings of a bio-film of some type. Any thing like that is bad news to one degree or another. It could be relatively benign or it could be the start of full on root rot.
One other critical element of keeping bacterial growth down in hydro is water temps. It really helps to keep the temps below 70F if at all possible. If you cant do that, then even if you are running pure salt nutes - with zero organic components as listed above - then you really need to run sterile. By that I mean you need to use low dose chlorine or H2O2 on a regular basis. Some peoplke have luck with bio based enzymes and stuff like hydroguard, but I think they are useless - especially if you have an established infection like in your case, or if temps are higher than 70F. For sure you cannot mix the two - chlorine or H2O2 will just kill the beneficial bacteria in hydroguard, etc.
