I use coco and I've never got them from brick coco. I have got em from bags of ready to use coco though. I think if you don't see them in the 1st couple weeks then they're probably coming from the environment. I do have them pop up a couple times a year. I think I bring them into the tent on my clothes from the wife's houseplants since she's always bringing new plants in and there's always gnats around her stuff, or the plants in the yard when it's warm enough outsideDo any of you guys 'cook' your media, the ones that use 'soil', for a period? I do that to fresh media especially. It comes from people saying they got it from new stuff.
I 'cook' my used media to just mainly get the roots eaten up by the microbes. I'll usually re-amend at the same time.