Can say this with my experience, controlling fungus gnats requires a lot of patience. We tried to get rid of these pests by keeping our place less humid and bringing in some predatory mites. This helped us a lot. But after few months we again faced this problem and this time we decided to consult the pest control in Sacramento professionals to know which chemical spray can be used to kill these houseplant insects. They suggested to use biological insecticide and repeat the application every five days until the gnats are eradicated.