Apply a houseplant fungicide, the kind you mix with water, or just use alone, but just to the soil, as if you were just watering.

You must be doing something really wrong. I've had them thrive in regular potting soil (in a well draining pot) or in normal bench-mix with no problem.

Bob

Bob - maybe you need to remind yourself on occasion that some people don't live in a hot, humid place where plants can just be thrown outdoors for life, but need to suffer our attempts indoors at creating the tropics. That wasn't a very nice comment.

It was an observation, I spent most of my life living in Pennsylvania, USA and grew some tropicals there. Even with some poor conditions in the beginning I don't remember one having root rot. So, something must be pretty wrong.

Bob

I have had it in many of my (previous) ficuses, actually. Do you think it might be spreading? Or is it sometihng that I myself am doing? I can't think of anything at the moment I might be doing wrong...

Watering too often, it's normal (if wrong, as Bob says).