I was going to suggest maybe giving them a hair cut. The thing is though if those roots are dead because of bacteria, its just gonna spread. I'm down in Australia, I can't find hygrozyme, but that stuff looks like the stuff you need. There is nothing wrong with trimming roots, its risky though since the roots which are still healthy are in small numbers. Have you tried adding more oxygen? Another of those air stones and run it on the opposite side of the bucket.
My pump is a 400LPH unit twin channel so 200 per channel. I run two air stones in the one bucket and from now on I will be oxygenating my future water changes. I got a new PH probe today. Originally I was on 6 with litmus paper. With the probe I'm on 6 to 6.1.
80F translates to what, 26C. My bucket has never peaked over 24.4C. Does your bucket smell at all?
My plant feels soft, a little wilty. Not sure of the cause, ph is good, roots look good, temps are good, ppms are now upped to 599. Other than the roots though, your plants look brilliant.