Keeping the terrarium sealed allows for the circulation of water, but terrariums must be opened once a week to remove excess moisture from the air and walls of the container. This is done to prevent growth of mould which could damage the plants and discolour the sides of the terrarium.Terrariums must also be watered occasionally, the absence of condensation on the walls of the terrarium or any wilting of the plants is an indicator that the terrarium requires water.