Closed Terrariums : Check the terrarium frequently for the appearance of condensation on the glass. If large water drops appear on the glass, the container should be left open for a while, until excess moisture evaporates. In some cases, it may be necessary to keep the lid slightly open to allow fresh air circulation. However, keep in mind that a balanced terrarium should have a certain amount of moisture on the glass. A completely enclosed terrarium requires little or no watering but when in doubt, always water less. If you can see water within the pebbles at the bottom, there is way too much water.