Fungus gnats are mainly attached to decomposing plant tissues of Haworthias and organic components in the growing medium. Adult fungus gnats don’t harm plants, but larvae can damage roots and therefore stunt plant growth, particularly in seedlings and young plants.
For an heavily infested plant, clean as much of the growing medium out of the roots and completely discard it. Some roots may be torn off but the plant will be fine. Meanwhile remove any rotten plant tissues. Soak the plant in 1% hydrogen peroxide solution for 15 minutes and rinse in water. This kills any eggs larve and fungus on which larve primarily feed on. If a major wound is created on the plant, follow the treatment after root pruning. Otherwise let the plant dry out in a shaded place before planting it in a fresh growing medium. For a less infested plant, add 1% hydrogen peroxide to the solution in the next several waterings. For remaining adult fungus gnats, place yellow sticky traps around the plants to capture them.
Using an pure inert growing medium which is resistant to decomposition is an excellent preventative measure.