Fungus gnats will die on their own very quickly, but they also breed very quickly. You will need to eliminate all sources of moisture except the actual water bowl. There may be other areas of your apt./house that could be a problem. Drop ceilings, potted plants, dishwashers,drip pan under refridgerators, any othe places that could collect water/moisture. Also check window frames, sometimes the collect moss and could attract gnats.