2 plants, same strain,nutes. 1 plant won't gr...

First of all, you shouldn't be touching the roots if you haven't sterilized your hands.... Second, it looks like something has contaminated that bucket... third, you can't expect to see any change in one day... so we'll go from there... I would do a couple of things here: first, I would do another bucket change and thoroughly clean the bucket with H2O2 before adding in new pH'd water. While the plant is out of the bucket, I would take it to the sink and spray the roots hard with the sprayer to dislodge as many dead roots as you can. Then I would take a gallon of pH'd water that has about 4tsp of H2O2 in it and thoroughly water all the clay pebbles and roots that are left to sterilize the pebbles of any contaminates that may be in there. Back to your bucket - Add 3tsp of H2O2 and 1tsp of RapidStart (General Hydroponics), and 1/4tsp each of FloraGrow, FloraMicro and FloraBloom. The only other thing I would consider is 1/4 tsp of calimagic or some calmag (they're pretty much all the same but calimagic is in the same nute line). The H2O2 will kill off any fungus or other contaminates, the RapidStart will get the roots going and the small doses of nutes will give the plant SOMETHING to eat while the roots try to repair themselves. One thing you need to resist when growing in DWC (and I know it's hard!) is looking at the roots. That bucket needs to be completely lightproof all the time. The one thing I'm NOT seeing in your bucket is a hole in the top through which you add water/nutes which suggests to me that you take the lid off frequently - probably daily? My buckets did not have that access hole either so I drilled one in each and cover it with duct tape... when I need to add water/nutes, the duct tape is lifted just enough to quickly add what I need and then is sealed right back up. I might have to do 3 or 4 bucket changes throughout a grow and that is the only time the lid is ever removed.... and every bucket change includes cleaning it with H2O2 and then adding those 3tsp into the fresh water along with any nutes. Don't give up on this lady even if she doesn't promise to be a great grow... I think you'll be able to get something out of her and you will learn a lot by trying to get her back in shape... and I don't see why you can't get her back in shape! She won't keep up with the other one but that's ok... she's a photoperiod, yes? That means you've got all the time in the world with her.. As for the nutes... go really easy on her for a little bit... no more than 1tsp each of the Flora series... resist the urge to check on the roots, too - you'll know when they're getting better because the plant will start to grow stronger... and you can go from there. Oh yeah - when you have that netpot off the bucket for the bucket clean/change, drill a 1" hole in the top of it and cover it with duct tape (either grey or black)... and then only open that hole up to add nutes. If you don't have a drainage hole at the bottom where you can extract water to check EC and pH, you can get a cheapo transfer pump (hand pump) like this one... https://smile.amazon.com/CWDRDX-Manual-Siphon-Liquid-Transfer/dp/B0BSBGWQKQ/ Good luck...