Fungus gnats are very common with mj, especially in coco. They don't do a lot of damage unless they get out of hand and start populating the root zone. Tap your pots on the sides and stir the top layer to see if several come out
BT ( mosquito dunks) will handle them as will a no pest strip. Sand is sometimes used as a top dressing to deter them as well as diatomaceous earth, but are not totally effective unless you put the pots in a bed of sand or DE also to keep them from going into the bottom holes.
Neem oil works as a preventative but doesn't do much after they are there