Keep Your Terrarium Balanced
In a completely closed system, the terrarium should need very little water. Moisture will collect on the insides of the glass and drip back down onto the soil and plants. If condensation becomes excessive, just open the lid to air it out for a day or two. When the plants need water, do so gently and sparingly. Even in a terrarium without a lid, err on the side of too little water rather than too much because there's no place for the water to drain. Keep the terrarium in bright light but out of direct sunlight, which can cause the temperature to build up inside the glass. Don't fertilize the plants regularly because you don't want to encourage rapid growth and fertilizer build-up, but if plants seem to need a boost, use a very dilute (1/4 strength) solution. Snip any leaves that touch the glass, and pinch the tips and trim plants as needed to keep them in bounds. You may need to replace a plant now and then.