The damping off. The stem rot. What are the water temps? It looks like you root in rock wool too, can I ask why you're using the DWC and not just rock wool? Anyway, what are the water temps of the DWC? Only think I could recommend is to clean it, run like 10:1 bleach through the pump and clean the whole thing. Then add water, your nutes or cloning stuff, whatever you use? and bleach to keep it sterile. If its already contaminated adding bleach aint gonna help. you gotta sanitize it and start over. If your water temps get around 80 or more you're gonna have to change it daily.
They aren't, but rapid rooters might be where you wanna start if you're having issues, I feel like the pre moistened rapid rooters and clonex gel is a good place to start if you're having cloning issues. but get that DWC cloner sanitized and see if that helps.