Thought I'd share a story because I am so glad my little experiment worked! 

I was struggling with this cattleya seedling for a few weeks. (I got it at a raffle, so pardon that it took me an embarrassing amount of time to realize that it was not a teeny miniature but in fact a seedling size orchid.) 

It was a new addition to my collection, so as with any orchid, I repotted it and kept an eye on it during quarantine. Before long I noticed these thin translucent strands of what must have been mycelium growing over the medium. Worried, I repotted again, removed dead roots, treated with peroxide, then watched. When the fungus reappeared, I repeated the repotting with fresh medium and used some Physan 20. This knocked it back for a while, but still not permanently. 

I pulled back on watering to try and control the spread, but it was a hard balance to strike because seedlings need more moisture than adult cattleya. If I let it dry out, the plant suffered and wrinkled. When I watered, I'd later see the fungus spread triumphantly. Damned if I do, damned if I don't, right?

Finally, it occurred to me while I was watering my other orchids. The rest of my collection has a pretty strong ecosystem of detritivores: springtails, orbatid mites, soil mites, etc. This newbie had nothing! 

I took one of the bark chips full of springtails from one of my older orchid pots and put it into the newbie's pot. And since then, I've never had problems with mycelium in its pot. It is now happily on the long road to recovery.

Got some mold? Try springtails.