Overwatering is the most common problem with terrariums and it generally leads to mold and bugs. Closed terrariums don't need water very often (Usually once or once every three months). As a rule of thumb, when you see condensation in your closed terrarium, there is no need to add more water. Do not allow water to sit at the bottom of the terrarium for extended periods of time as this may cause plant roots to rot. You can use paper towel to soak up excess water. Also remove the lid and place in a brighter area for a few hours (but never in direct sunlight). Regular filtered tap water, distilled water, or carbonated water can be used to water terrariums.