A terrarium is a miniature environment in which plants live and grow. Closed terrariums have a cover that allows the container to become a biosphere where water is produced by the transpiration of the plants and does not usually need any additional watering. A 10 gallon fish tank or aquarium is perfect for this type of terrarium.
Wash the aquarium out with dish soap and water. You should wash it even if it's new and certainly if it has housed fish before. Rinse thoroughly and leave no soap residue in the aquarium. Make sure it is dry before continuing.
Lay a thin layer, about ½ to 1 inch, of pebbles or non colored aquarium pebbles at the bottom of the aquarium. This will ensure good drainage around the roots of the plants so they will not get too much water. It also will help in deterring any mold.
Sprinkle a thin layer of crushed activated charcoal over the top of the pebbles. This will allow the water to remain clean as it eventually recycles in the environment. Open terrariums do not need this step because they are continually watered with new water.
Place a layer of sphagnum or Spanish moss after the charcoal and before the soil. This separates the soil and does not allow it to sink down into the pebbles where it can become muddy. The roots of the plants will be able to penetrate and grow through the moss. This is an optional step but one well worth doing.
Lay down a thick layer of sterile potting soil of about 4 to 6 inches. This layer can be thicker if you like to make contours of hills and valleys. Try to make it no more than 8 inches and no less than 3 inches.
Choose smaller houseplants to plant in the terrarium. Desirable plants do not grow large to begin with. Choose plants with the same water and sun requirements as it will be difficult to separate them. Never plant cacti with houseplants. Choose a variety with different shapes and colors. Some examples are palms, small pines, ivy, prayer plant, and crotons but there are many that can be used in a terrarium. Pick an odd number of seven or nine for a 10 gallon aquarium.
Plant the plants by digging a hole that will accommodate the root ball of each plant in the soil. Remove the plant from the pot and loosen the roots. If there are some long roots you can cut those with clippers or scissors so they are all uniform. Place the roots in the hole and cover up to the base of the plant with soil. Tamp down lightly so they are secure in the soil. Once all plants are in, spray with water in a spray bottle but do not saturate the soil.
Place a piece of glass or hard plastic over the top of the aquarium. You can have a piece cut to size at a glass store. You will have to remove it if the terrarium needs to be watered. Place the terrarium in a place with filtered sunlight.