Do you think the brown stuff could be a product you’ve used like the voodoo juice? It seems quick for young roots to get that bad that soon especially when you’ve been on the ball with res changing and keeping an eye on your temps. Root rot will cause damage and not wash off and seeing as you’ve got white roots now I’m not so sure it’s rot. Now you’re using h2o2 it shouldn’t be a problem anymore if it is but you wont be able to use biological nutes so check what you’ve been using as not to waste them. I know
@ProfessorFlora is using mega crop alongside h202 and getting great results. Looking at your plants I’d give them some calmag to start with and see how they respond over the next 12 hours. Beauty of dwc is you know very quickly if the plant is happy or not so experiment with adding small amounts of nutes as every strain is different. If things go bad just start again with a fresh res.