Fungus gnats do feed of fungal growth in the soil of plants. Their presence usually is a clue that you’re keeping you plants too moist. Let your plant get very dry between watering and NEVER let water stand in the saucers below the pots for more than a few minutes. You want to water heavily then let the plants get nearly bone dry before watering them again. Let them start to wilt before you soak them again. Drowning is the #1 cause of houseplant death. As the days get shorter and the windows are closed for the winter, plants inside will need a tiny fraction of the water they’ll use during the long days of summer. Back off on the watering and stop feeding.
You should go to Hewitts and get some systemic insecticide for houseplants. It will kill any insects in the soil then grow up into the plants and kill any insects that are attacking the plants.