Fans are very important. Mine only run during the day-light hours but I think my good air movement prevents mold and mildew, and dries my plants after fertilizing/watering. I have never had any pests (scale, aphids, etc) on any of my plants as far as I know, and I think the air movement (perhaps) prevents those infestations. Since many of my plants stay outside during the mild months, they sometimes will get ants, but that's about it. Oh, those pots on the ground will sometimes/often have earthworms, but that's a good thing. The only time I get rid of them is when I am shipping my plants bare-root.