How to Make a Terrarium Out of a Fish Tank

There are two important options for your terrarium design: leaving the top open or closing it with a fitted glass or plastic cover. Decide which type you want to create before you begin since different plants may fare better in each type of environment.

Whether the aquarium is new or previously used to house your fish, it needs to be cleaned out. Dust and bacteria are present even on new aquariums, so you'll want to give your plants a clean, fresh start. Use mild liquid dish soap and water with a sponge, if needed, to scrub away any algae or scum. Rinse thoroughly to make sure there is no residual soap and dry completely before building the terrarium.

Line the bottom of the fish tank with gravel, small pebbles, rocks, or aquarium pebbles. This will function as a drainage system to prevent the plants' roots from rotting, similar to the way holes at the bottom of flower pots work. It will also help prevent the growth of mold. You can add a layer of decorative, colored pebbles over the top if desired. Aim for approximately 1/2 to 1 inch in depth.

If you are creating a closed terrarium, dust the pebbles with a thin, approximately half-inch layer of horticultural charcoal, activated charcoal, or activated charcoal pebbles. This will help keep the water clean as it is recycled in the terrarium. It will also help prevent accidental overwatering since it is an absorbent material. If you are creating an open terrarium, this step is not necessary since you will be watering it frequently with fresh water.

Add some sphagnum moss, commonly referred to as peat moss. Do this after adding the charcoal and before adding any potting soil. This will prevent the soil from falling below the pebbles and creating mud. The plant roots will be able to grow into the moss layer. It also adds some color and variety to the ecological system.

Next, you want to apply a thick layer of high-quality potting soil that's suitable for the types of plants you'll be using. You can premoisten the soil if desired. Add several inches of soil so that the roots can take hold; however, you could make it a little thinner or thicker, depending on the plants you are using and the size of your terrarium. You can create a scaled-down landscape with tiny peaks and valleys depending on your design. Use your imagination and creativity.

Now rehome the plants in the terrarium. Get each plant ready by removing it from its pot or container and gently loosening the roots. If some of the roots are very long, trim them carefully with a clean, sharp pruner or plant scissors. Make holes in the soil large enough to create a rooting zone, similar to how you transplant potted plants to an outdoor garden bed. Cover the roots and base of the plant with soil to about the same level as they were planted in their pots and pat down gently so they are held firmly in place.

Rather than watering the soil directly, aim a stream of water from a spray bottle toward the inside walls of the terrarium so it runs down each wall into the soil. The soil should feel moist when you test it with your fingers, but don't soak or oversaturate it, which could end up killing your plants. Note that a closed terrarium may only need to be watered every three months or so, unless you notice that it seems dried out.

Add driftwood, branches, river rocks, miniatures, small statues, or tiny toys — anything you fancy as long as it can safely get wet. You can even create a small-scale garden or park with stepping stones, bridges, pagodas, houses, and castles. If you have any aquarium decorations, you can use them as well.

If you decide to make a covered terrarium, then use either glass or hard plastic that fits on the top. Make sure it can be easily removed if your plants need watering or tending to. If creating an open terrarium, no cover is needed, but you will likely need to water more frequently.

Terrariums need indirect or filtered sunlight. Do not place it in direct sunlight since this may provide too much sun and heat. You can place the terrarium on a table or plant stand near a window in your home. You may need to rotate the terrarium from time to time to prevent the plants from growing at an angle toward the light. Or you can use a grow light instead.