Hi mate, Hydrogen peroxide reportedly kills fungus gnat eggs, larvae, pupae, and adult flies on contact. Neem oil doesnt actually kill them, they just dont like it.
be sure you’re using the typical 3% household hydrogen peroxide, not the industrial-strength stuff! Dilute it down slightly and mix 1 part 3% hydrogen peroxide to 4 parts water (e.g. 1 cup peroxide and 4 cups water).
Allow the soil to dry slightly before application. Then, either spray the surface of the soil thoroughly until it saturates down a couple of inches. Or, for a deeper and more effective treatment, water the entire plant with it – soaking all of the soil. It will fizz and foam, which is totally normal. It quickly breaks down into molecules of water and oxygen.
Hope this helps 🙂