Actually, healthy roots and healthy plants do not die and rot attracting fungal outbreaks, but clearly something real bad happened real fast to your roots.
I see a lot of black stuff inside your air tubing, what is that? Air tubing is cheap @ pennies a foot, maybe change tubing out eh.
I have heard and read that Pythium and other fungi love hydroponic environments that develop low oxygen problems. Their anaerobic colonies thrive quickly when a low oxygen conditions present, fungi are opportunist and ubiquitous in hydroponic solutions just patiently waiting for a feast of dying, rotting roots to eat and the outbreak is ON.
So what in the world could have happened that killed your roots so quickly?
Looks like you have already tested everything you can think of in you nutrient water that can kill your roots so quickly like this.
Have you considered tested your DO Saturation in your nutrient water at the onset of this crisis just to confirm or deny that your roots may have simply suffocated, died and rotted?
have you did check this out, what was your DO in that nasty reservoir water containing those rotting roots?
Is your nutrient DO 100% Saturation or greater (safe or not safe)?
If your DO is low, your roots have most likely suffocated from lack of oxygen.
If your DO test reveals low oxygen is a problem, well, the most logical thing to do is increase the DO in the water and that problem is totally resolved, fixed and low O2 root suffocation is totally eliminated.
A simple solution that is fixable ASAP. Suffocation is a real killer.
*Understanding Dissolved Oxygen by Kurt Becker 2/27/2017 http://www.ballpublishing.com/GrowerTalks/ViewArticle.aspx?articleid=22058
Kurt’s a real Guru and claims, “… 8 PPM DO Concentration is good – 30 PPM DO Concentration is much better. And DO Supersaturation is best, “it can increase quality and plant growth, reducing cropping time and overall health. High levels of dissolved oxygen promote healthy root growth.”