I'm not liking that dark area on the roots. It's normal to see them get tan as they age tho the ends that are constantly growing should be white and fuzzy with root hairs.
I'd dose them good with food grade peroxide if you can get it. I just picked up a litre of 35% at the health food store tho have a half one here. Before making a DIY chiller I always gave them 0.5ml/L twice a week to prevent rot starting but I'd use at least 2.0ml/L every 2nd day for 3 or 4 doses to kill what may be starting. Even higher doses daily can be used if rot has taken hold.
I've never used beneficial bacteria stuff in DWC. From years of reading all sorts of stuff about growing it seems to work if used from the start to prevent rot but isn't going to save your plants once rot has set in. I helped a guy that had bad rot a couple years ago on the now defunct Cannabis Culture forum and told him to use 4ml/L daily for a week and it worked. He had serious rot going on when he reached out for help but his plants survived and he got a great crop.
I'd get on that PDQ as rot can take over in a day or two.