Oxycloner, roots in 9-12 days, only water, changed out at 7day intervals. Dipped with
clonex and 8days in i use vitamin b1. The trick is to keep it clean. Under 19w led, cheap Home Depot grow light in a 1x2x4 tent just for cloning. Been using for a while. I do change hoses out about once a month, they are cheap. , I have spare air stones that I soak in peroxide and switch out with water change. And cleaning is just soap water , spray with peroxide, rinse and plain ol tap water works .
As far at the rot, I think it maybe bacterial and or lack off leaf material. Try not trimming it back.
I monster crop so these are all flowering lower cuts. If I do normal cuts I feed the Mother flowering light nutes for a week before I take cuts. They root better when the nitrogen in them is lower..