A terrarium is a miniature ecosystem contained in a small environment, such as a glass fish tank. Mushrooms may sprout up in a terrarium on occasion. Unfortunately, the only way to kill mushrooms in a terrarium without killing any other plants is to replace the soil, because the fungus that is causing the mushrooms inhabits the soil. One can, however, attempt to dry out the terrarium first.
Open the lid of the terrarium and remove the mushrooms.
Allow the entire terrarium to dry out for a few days, or until the plants begin to wilt. By removing excess humidity, you may be able to kill the fungus in the soil without killing the other plants.
If the mushrooms do not stop growing, uproot the plants from the terrarium and place them aside, keeping their roots out of direct sunlight if possible.
Empty the mushrooms and all of the dirt from the terrarium into a garbage bag and discard.
Fill the terrarium halfway with a solution of one part bleach and four parts water, and use the sponge to scrub the terrarium clean. Use the same sponge to clean any light fixtures, rocks or the lid of the terrarium.
Rinse all terrarium components until no smell of bleach remains.
Replace the soil and decorations in the terrarium.
Rinse the plants carefully, ensuring there is none of the old contaminated dirt left on them, then replant them in the terrarium.