What Domenik and Nocone_Purple wrote, just Rhizotonic staining..........no evidence of dead roots at all.

Looks more like nutrient staining from Rhizotonic than root rot temps (19°C), pH 5.8 and EC 1.4 are on point, and since the roots don’t look slimy or mushy, you’re likely fine

That's dead roots. If you do nothing root rot will develop and your plant will perform poorly or even die. I would Take a Sprayer with water and hydrogen peroxide and spray out the brown and dead material with some pressure.when it's gone,let the roots sit in water with hydrogen peroxide for 2-3 hours (pump on). After that mix a new solution.

Hey, sorry the response will be a bit long. The hypothesis about Rhizotonic: It's very likely. Organic additives or root boosters like Rhizotonic often stain the roots brown/blackish, especially on the upper part that isn't constantly submerged but sprayed by the bubbles. If the roots are firm and don't slip between your fingers, it's probably just a coloration. The temperature (19°C): At 19°C, you're in the perfect safety zone. Pythium (root rot) has a hard time developing below 21-22°C. So, on this point, I'm pretty calm. Oxygen: The fact that it's foaming a bit shows that your air stones are doing their job. However, if the foam is too thick, it can sometimes smother the base of the roots a bit. What I'm going to check to be sure: The smell: This is the ultimate test. If it smells like fresh soil or nothing at all, it's Rhizotonic. If it smells like swamp or rotten potato, it's a bacterial infection. The texture: I'm going to touch the dark roots. If they're solid, that's good. If they stay in my hands or are slimy, it's the beginning of root rot. The tips of the roots: If the tips of the roots (lower in the water) are white and healthy, then the black coloration at the top is indeed due to the product deposit. I hope I could help you, have a good evening.