Use can wash the soil with a hydrogen peroxide and water mix. You will have to remove the plants from the cage.
use one part hydrogen peroxide
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To three to four parts water

Pour over the top of the potting soil and let drain. You should see and hear bubbles at the surface of the soil.
The hydrogen peroxide kills the larva, egg and fungus in the soil that the fungus gnats are attracted to.
let the plant drain completely and the top of the soil dry before placing back in the cage.
this has worked for me for years in all my hours plants and chameleon plants.
fungus gnats can live in soil for years as eggs and will hatch when the soil is inside.