The basic setup: tank size, substrate, and water

A Terraria aquarium starts with a glass or acrylic tank at least 10 gallons, though 20 gallons gives you more room to work with plants and fish without constant maintenance. Fill it with dechlorinated water — tap water left sitting for 24 hours, or water treated with a dechlorinator product from any pet store. The substrate (bottom layer) should be aquarium-grade gravel or sand, about 2 to 3 inches deep, rinsed thoroughly before you add it so the water doesn't turn cloudy.

Rinse the gravel in a bucket under running water, stirring it with your hand, until the water runs clear. This removes dust and debris that will cloud your tank for weeks if you skip it. Add the gravel to the empty tank, then pour water slowly over it — aim the stream at a plate or bowl sitting on top of the gravel so the force doesn't scatter it everywhere. Fill the tank three-quarters full at this stage; you'll add more water after plants and decorations are in place.

Key Takeaways

  • A 10-gallon tank minimum with 2 to 3 inches of rinsed gravel substrate will hold most beginner aquarium setups without constant water changes.
  • Live plants like Java fern and Anubias reduce algae and provide oxygen, but they need 8 to 10 hours of light daily from an aquarium light fixture, not just room light.
  • A filter rated for your tank size keeps water clean; a heater set to 72 to 78 degrees Fahrenheit prevents temperature swings that stress fish.
  • Stock fish slowly — add only a few at a time, wait a week, then add more — so the filter bacteria can keep up with waste.
  • Test water weekly for ammonia and nitrite (both should read zero) using a liquid test kit, not strips, because strips are unreliable.

Lighting and plants: what actually grows in an aquarium

Live plants are the difference between an aquarium that stays clean and one that turns green with algae. Java fern, Anubias, and Amazon sword are the easiest to keep because they grow slowly and tolerate low light. Tie Java fern and Anubias to rocks or driftwood with fishing line — do not bury the rhizome (the thick horizontal stem), or it will rot. Plant Amazon sword directly in the gravel with the crown (where the leaves meet the roots) just above the surface.

Plants need light to photosynthesize and produce oxygen. Room light alone is not enough. Buy an aquarium light fixture rated for your tank size — a 10-gallon tank needs a 15 to 20-watt LED light, a 20-gallon needs 25 to 30 watts. Run the light 8 to 10 hours a day on a timer. Without a timer, you will forget and leave it on 16 hours, which causes algae blooms. With a timer set to turn on at 8 a.m. and off at 6 p.m., the plants grow steadily and algae stays under control.

Filtration and water temperature: keeping the tank stable

A filter removes waste and dead matter that would otherwise poison the fish. Buy one rated for your tank size or slightly larger — a 10-gallon tank needs a filter rated for 10 to 20 gallons per hour (GPH), a 20-gallon needs 20 to 40 GPH. Hang-on-back filters are cheapest and easiest to maintain. Sponge filters are quieter and gentler on small fish. Do not use a filter rated for less than your tank size; it will not keep up with waste.

A heater keeps water temperature steady between 72 and 78 degrees Fahrenheit, which is the range most aquarium fish tolerate. Without a heater, room temperature swings stress fish and make them sick. Buy a heater rated for your tank size — a 10-gallon tank needs a 50-watt heater, a 20-gallon needs 100 watts. Set it to 75 degrees and check the thermometer daily for the first week to make sure it is holding steady.

Cycling the tank before adding fish

Before you add fish, the tank must cycle — meaning beneficial bacteria must colonize the filter and gravel to convert fish waste (ammonia) into less toxic compounds. This takes 4 to 6 weeks. During cycling, ammonia and nitrite will spike and then drop to zero as bacteria build up. You can speed this up by adding a bottle of bacterial starter culture (sold as "Tetra SafeStart" or similar) or by moving filter media from an established tank.

Test the water every 2 to 3 days with a liquid test kit — not test strips, which are inaccurate. You are looking for ammonia and nitrite to both read zero, and nitrate to read between 5 and 20 ppm (parts per million). When all three readings are stable for 3 days in a row, the tank is cycled and ready for fish. If you add fish before cycling is complete, ammonia will spike and poison them.

Choosing and stocking fish: start small and go slow

Stock the tank slowly so the filter bacteria can keep up with the waste load. Add only 3 to 5 small fish the first week — neon tetras, guppies, or corydoras catfish are hardy choices. Wait a week, test the water, and if ammonia and nitrite are still zero, add 3 to 5 more. Repeat this process every week until you reach your target number. A 10-gallon tank holds about 10 small fish; a 20-gallon holds about 20.

Avoid large or aggressive fish in a small tank. Goldfish, plecos, and cichlids grow too big or produce too much waste for a beginner setup. Stick with small, peaceful species that school together — tetras, rasboras, and danios — or bottom feeders like corydoras catfish that clean up uneaten food. Never add fish from different bags at the same time without acclimating them first. Float the bag in the tank for 15 minutes, then slowly add tank water to the bag over 30 minutes, then net the fish into the tank.

Water changes and maintenance: the weekly routine

Change 25 percent of the water every week. Siphon out water using a gravel vacuum — a tube with a bulb at the top that you squeeze to start the siphon, then let gravity pull water into a bucket. As you siphon, move the vacuum over the gravel to remove waste that settles there. This keeps ammonia and nitrate from building up between changes. Replace the water you removed with fresh dechlorinated water at the same temperature as the tank.

Clean the filter cartridge once a month by rinsing it in old tank water (not tap water, which kills the bacteria). Wipe algae off the glass with a magnetic scraper or soft cloth. Check the heater and thermometer daily for the first month, then weekly after that. If algae turns the water green, reduce light to 6 hours a day for a week and do a 50 percent water change. If it comes back, you are overfeeding — fish should eat all food in 2 to 3 minutes, no more.

Decorations and hardscape: rocks, driftwood, and caves

Rocks and driftwood anchor plants and give fish places to hide, which reduces stress and makes them more active. Lava rock, slate, and smooth river rocks are safe for aquariums. Driftwood should be aquarium-grade or boiled for 30 minutes to remove tannins (which turn water brown). Avoid sharp rocks and anything painted or treated with chemicals. Arrange hardscape before you add plants so you can bury plant roots and tie plants to rocks without disturbing the layout later.

Caves and hollow decorations are especially useful for fish that hide during the day — corydoras catfish, plecos, and some tetras. You can buy ceramic caves or make your own by stacking slate pieces. Make sure any caves have smooth edges and are large enough that a fish can turn around inside. Avoid plastic decorations with sharp edges or small openings that fish can get stuck in.

Frequently Asked Questions

How long does it take to set up a Terraria aquarium?

The physical setup — gravel, filter, heater, light — takes 1 to 2 hours. The cycling process, where bacteria colonize the filter before you add fish, takes 4 to 6 weeks. Plan to wait at least a month before the tank is ready for fish.

Can I use tap water straight from the faucet?

Tap water contains chlorine, which kills fish and beneficial bacteria. Let it sit for 24 hours before adding it to the tank, or use a dechlorinator product from a pet store. Both methods remove chlorine safely.

What should I feed my fish and how often?

Feed once or twice a day, only as much as the fish can eat in 2 to 3 minutes. Overfeeding is the most common beginner mistake — uneaten food rots and fouls the water. Use a small pinch of flake food or pellets, not a handful.

Why is my water cloudy after I set up the tank?

Cloudy water usually means you did not rinse the gravel enough before adding it. Do a 50 percent water change and rinse the gravel more thoroughly next time. Cloudiness from bacteria (a white haze) clears on its own within a few days as the filter catches particles.

Do I need a water test kit if the fish look healthy?

Yes. Fish can look healthy while ammonia or nitrite is slowly poisoning them. Test weekly with a liquid kit to catch problems before fish get sick. Strips are cheaper but unreliable — a liquid kit costs more upfront but gives accurate readings.