Fungus gnats often appear when the terrarium environment is kept too wet, says Katie Palmowska of Tropical Glass. 'If you notice them, it's essential to open the terrarium to air it out and allow the soil to dry. Adding a layer of gravel on top of the soil can also help prevent their appearance, as it reduces the surface moisture that these pests are attracted to.
'If you find a significant number of fungus gnats, consider using yellow sticky traps,' Katie continues. 'Place them inside the closed terrarium, where they'll attract and capture the gnats.'