Sorry, what's de? 

Sulphur seems to take care of most surface molds that I've encountered, for whatever that's worth.  I think the gnats like to eat molds and such.

You're right though- the solution that seemed to work best for me was less watering and using better-draining soil, i.e. more sand in the mix.  Thanks yet again for all your help.  You're a great resource and a generous person!