What You're Building and Why an Aquarium Works

A terrarium in an aquarium is a self-contained landscape for plants and small animals that lives inside a glass tank. The aquarium's sealed or semi-sealed environment traps moisture, creating the humid conditions that tropical plants need. Unlike a traditional aquarium, there is no water—instead, you layer soil, rocks, and plants to mimic a forest floor or wetland.

An aquarium is ideal for this because the glass walls let you see the whole ecosystem, the shape gives you room to arrange plants at different heights, and the existing frame means you don't need to build a container from scratch. A 10-gallon or 20-gallon tank works well for a beginner terrarium.

Key Takeaways

  • Layer your terrarium from bottom to top: rocks for drainage, activated charcoal to prevent odor, potting soil, and then plants and decorations.
  • Choose plants that thrive in humid, low-light conditions, such as moss, ferns, pothos, and prayer plants.
  • Mist the soil lightly once or twice a week, or seal the lid to let the terrarium recycle its own water through condensation.
  • Place your terrarium in indirect light away from heating vents and direct sun, which can cook the plants or cause too much condensation.
  • Check the soil moisture by touching it with your finger—it should feel damp but not soggy, and roots should never sit in standing water.

Gather Your Materials and Tools

You will need an aquarium (10 gallons or larger), a lid or piece of glass to cover it, small rocks or pebbles for the bottom layer, activated charcoal (sold in pet stores or online), potting soil, and plants suited to humid conditions. You will also want a spray bottle for misting, a small shovel or spoon for moving soil, and tweezers or tongs to position plants and rocks without reaching too far into the tank.

Optional but helpful: moss poles or cork bark for climbing plants, small pieces of driftwood or slate for visual interest, and a small watering can with a narrow spout. Do not use garden soil or soil from outdoors—it compacts, drains poorly, and may contain pests or fungi that thrive in the sealed environment.

Layer the Tank From Bottom to Top

Start with a 1-inch layer of small rocks or pebbles at the very bottom. This allows water to drain away from the soil and prevents roots from sitting in moisture. Pour the rocks in and spread them evenly across the tank floor.

Next, add a thin layer of activated charcoal—about half an inch. Charcoal absorbs odors and prevents mold from building up in the sealed environment. Sprinkle it over the rocks and spread it with your spoon.

Now add 2 to 3 inches of potting soil on top of the charcoal. This is where your plants will root. Dampen the soil lightly with your spray bottle as you add it so it settles and compacts slightly, but do not soak it. The soil should feel like a wrung-out sponge.

Once the soil is in place, you can begin planting. Use your tweezers or tongs to position plants, and press soil gently around their roots. Arrange taller plants toward the back and shorter ones toward the front so you can see the whole landscape.

Choose Plants That Thrive in Humid Terrariums

Moss is the easiest choice and often the most attractive—it requires almost no soil depth and adds texture. Ferns, especially maidenhair and button ferns, love humidity and low light. Pothos (also called devil's ivy) grows slowly in a terrarium and trails beautifully. Prayer plants have striking patterned leaves and stay compact. Peperomia and African violets also do well in humid, indirect light.

Avoid succulents and cacti—they rot in humid conditions. Do not use plants that need bright direct sun, because the glass will magnify heat and cook them. Stick to plants labeled as tropical houseplants or shade-loving plants. Start with three to five plants in a 10-gallon tank so they have room to grow without crowding each other.

Decide on Open or Closed and Water Accordingly

An open terrarium has no lid. You mist the soil lightly once or twice a week with your spray bottle. Water evaporates, so you need to add moisture regularly. This works well if you want to avoid mold or if you like the ritual of misting. It is also easier to reach in and adjust plants.

A closed terrarium has a lid or glass cover. Water condenses on the glass walls and drips back into the soil, creating a cycle that needs almost no watering from you. Check the glass after a few days—if it is completely fogged, crack the lid open for a few hours to let some moisture escape. If the soil looks dry, mist lightly once and close it again. Closed terrariums can develop mold if they stay too wet, so monitor them closely for the first two weeks.

Most beginners find an open terrarium easier to manage. You have more control, and mistakes are easier to fix. Once you understand how your plants respond, you can experiment with closing the lid.

Position Your Terrarium and Watch for Common Problems

Place the tank on a shelf or table in indirect light—a north-facing window or a spot a few feet from a south-facing window works well. Do not put it in direct sun, which heats the glass and can scald the plants. Do not place it near a heating vent, air conditioner, or drafty door, because temperature swings stress the plants and dry out the soil unevenly.

Check the soil once a week by pressing your finger into it about an inch deep. It should feel damp but not wet. If water pools on the surface or the soil smells sour, you are watering too much—let it dry out for a few days and crack the lid if it is closed. If the soil feels bone-dry and the plants look droopy, mist more often or add a little water with your watering can.

Watch for mold or algae on the glass or soil surface. A little white fuzz is normal and harmless, but if it spreads, reduce watering and increase air circulation by cracking the lid open. If a plant dies, remove it promptly so it does not rot and spread fungus to the others.

Maintain Your Terrarium Over Time

Terrariums need very little maintenance once they are established. Mist or water every one to two weeks depending on whether the tank is open or closed. Pinch off dead leaves and trim plants that grow too large or shade out their neighbors. Every few months, check that the charcoal layer is still visible and not buried under decomposing plant matter—if it is, you may need to gently remove the top layer of soil and replace it.

Most plants in a terrarium grow slowly because of the low light and high humidity. This is normal and actually desirable—slow growth means less maintenance. If a plant becomes leggy or stops growing, it may need brighter light. Move the tank closer to a window or add a small grow light a few inches above the plants.

Frequently Asked Questions

Can I add small animals like frogs or insects to my terrarium?

Yes, but only if you research the species first. Dart frogs, tree frogs, and isopods (pill bugs) do well in humid terrariums. However, they need specific temperatures, food, and hiding spots. Start with plants alone until you are confident the environment is stable, then add animals if you want to.

What if my terrarium gets too much condensation and the glass is always fogged?

This means the soil is too wet or the air inside is too humid. If the tank is closed, open the lid for several hours each day until the glass clears. If it is open, reduce misting to once a week. You can also wipe the glass with a soft cloth to improve visibility while you wait for conditions to balance.

How long does a terrarium last?

A well-maintained terrarium can last for years. Plants grow slowly and live a long time in the humid, stable environment. Eventually, some plants may outgrow the space or die naturally, but you can replace them. The soil will break down over time and may need refreshing every two to three years.

Can I use tap water to mist my terrarium?

Tap water is fine for most terrariums. If your tap water is very hard or heavily chlorinated, let it sit in an open container for 24 hours before misting so the chlorine evaporates. Distilled water works too but is not necessary unless you notice mineral buildup on the glass.

What should I do if moss or algae covers the glass?

A light coating of algae is harmless and actually a sign the environment is healthy. If it bothers you, wipe the glass gently with a soft, damp cloth. Prevent heavy algae by keeping the terrarium out of direct sunlight and not overwatering. If mold appears instead of algae, increase air circulation by opening the lid more often.