Possible root-rot on my Kush. Growing in soil. Any suggestions for soil growing root rot. The soil I changed was moist.

If u didn't let it dry up. Or kept the soil to wet throughout the grow, then your roots have had it. Soil needs to dry up nearly fully. It helps roots breath.. The plant is gone

Very difficult to recover and the outcome wouldn't be so satisfing i guess.. Better restart a new grow in this case.. Is only my opinion....

A cellulase enzyme is whats needed to treat root rot. Like Hygrozyme, Sensizyme, I've never seen advanced root rot so i can't speak to whether your plant suffers from it or not. When you said your leaves turned black i thought right away it was severe nutrient burn, but your plant displays many symptoms.

More for the future than right now unfortunately, I've never found anything to bring a plant back from advanced root rot. Use fabric pots, they allow air to pass through which dried the medium quicker speeding and air prunes the roots so they don't become as bound as in fabric pots. Lift your pot up, if it feels light give it a water, if it's still heavy don't water it Fabric pots allow more room to over water because they drain more effectively and dry out quicker, sorry I couldn't be more help for now but this has helped me over the years, just standard practice for me now, much easier doing it by eye with Coco coir aswell