I had potting soil fly infestation... so I treat them with black soap for the larvae, sticky cardboard for adult, then later with sand to eradicate these beasts. It work well, maybe i watered a little to much. Be careful to that!
I started to feed them with organic fertilizers from the biobizz brand (bio grow, bio bloom, top max) by following their usage table to the letter (by under dosing a little)