Fungus nats eat dead root matter and organic matter. If you water too much with organic matter they come to life. I use a compost tea with mycorrhizal and subcultures on my roots to help keep them away. Outside they get killed off in winter, but if you have a lot of them its from unhealthy roots. Not sure if you use a compost tea, but if you want I can give you a good formula to brew some. Its fairly inexpensive and easy to make.