Fungas gnats eat up the hairs on the roots of your plants and in doing so the plant is eventually unable to take up water.
I read that one of the most effective cures for fungus gnats is to drench the soil with a mixture of a teaspoon of non grease-cutting dish soap to a gallon of water/about 3,7 litres. If you put your potted plant up on blocks or bricks so that the water can run freely through the bottom holes, then slowly pour the entire mixure of soap and water through the soil. Let it drain until it isn't dripping anymore, then let it dry out as much as possible before watering again. This should kill any gnats or larvae that are in the soil. It's best to repeat the treatment in ten days to be sure you got them all.