You need a clean airflow coupled with surface scraping and old food removal.
Ideal case is to have an air pump with a 3 way spliter and 3 airstones, two underground at either side of teh nest and one above ground, this will keep the air clean above and below ground.
You have to surface scrape every week, not many people do this but it works, them little japanease stone gardens you get, get a mini rake from one and put fine mesh around thebottom of it to make like a net, run this above the surface of your sand or soil, about 3mm from ground level, this will pick up any mold that forms as to start with its very hard to see, you can try to rake the sand if your ants holes arnt in the way, turning it over inhibits mould!
ALWAYS take out old food, they wont eat it and its just sits and rots. most mould can form at 21oC>= so keep it below that temp if you can otherwise wait for hibernation, mould dies in cold weather :)
Remeber Clean Air, Clean Surface, Clean Dishes:
CACSCD
Regards,
Nixy