Hi!
New here, but not new to houseplant keeping. My house looks like a jungle and I'm very happy with it. There are several scientific studies about the happy times that plants bring to people, and that makes sense on so many levels. I do forest bathing (Shinrin-yoku in Japanese) every day - because even tough my house looks like a forest it lacks the sun and fresh air and all the wildlife that is encountered in the forest. The only way for me to be even happier is if I had some plot of
land of my own, but I don't. :c
Anyway, the fungus gnats. I am a student, I don't have the money or the patience to go around shopping for fungicides and herbicides, I actually prefer to learn what I'm doing wrong (even if that takes more time) and trying to solve it at home at the lowest cost possible - even if it means the plant will die.
So, I had many gnats in the past and the best thing you can do is repot the infected plant (or plants), and in my case I used that soil to put in my plants that are outside, because the gnats hate the fresh air. Then you give a thorough clean both to the plant and the pot it was in, put the new soil. Here comes the most important information, which actually serves for any other plant problem in any context: you have to take good care of your plant to prevent problems like these. It's easier, less expensive (both in work and financial terms) to do this instead of having to solve the problems later. In the case of fungus gnats, the best you can do is re-learn how to water the plant. Fungus gnats usually form when you are giving too much water to your plant. They
feed off the fungi that thrive on the excess moisture. While you're still learning how much water your plant needs, you can top up the soil with some sand. The sand lets the water pass through more easily, so the top of the soil will be drier and less prone to egg-laying from the gnats.
The other options: cooking the soil at a low temperature in the
oven or sticky traps (more info on that here
https://www.almanac.com/pest/fungus-gnats) or just throwing away the soil (last case scenario, never used it because I hate waste). I used traps but they are inefficient on a huge infestation, which was my case. So try to take good care of your plants first, you won't have to worry much about gnats, fungi or diseases if you do that. Try the App Planta to help you schedule watering of your plants. After some time, you'll become a pro!
Good luck.
Darya