Start with tank size, location, and the equipment you'll need before adding water

Setting up an aquarium means choosing a tank, placing it on a stable surface, installing a filter and heater, filling it with water, and letting it run for one to two weeks before adding fish. The order matters because fish need a cycled tank — one where beneficial bacteria have grown enough to handle their waste. Rushing this step kills fish. A 20-gallon tank is easier to maintain than a 10-gallon one and costs only slightly more, so most beginners do better starting there.

Your tank location determines whether the setup will work long-term. Place the tank on a sturdy stand or table that can hold its full weight — a 20-gallon tank weighs roughly 170 pounds when filled. Keep it away from direct sunlight, which causes algae blooms, and away from heating vents and air conditioners, which cause temperature swings. Make sure you can reach the back and sides for cleaning and maintenance. A spot near an electrical outlet is essential because you'll need power for the filter, heater, and light.

Key Takeaways

  • A 20-gallon tank on a sturdy stand placed away from sunlight and temperature swings is the most forgiving setup for a beginner.
  • You need a filter rated for your tank size, a heater set to 76–78 degrees Fahrenheit, and a light on a timer before you add fish.
  • Fill the tank, run the filter and heater for one to two weeks, and test the water with a kit that measures ammonia, nitrite, and nitrate before introducing any fish.
  • Tap water contains chlorine that kills beneficial bacteria, so treat it with a dechlorinator before adding it to the tank.
  • Gravel, plants, and decorations give fish places to hide and help bacteria colonize the tank faster.

Choose a filter and heater matched to your tank size

The filter removes waste and dead matter from the water and provides a surface for beneficial bacteria to grow. Buy a filter rated for a flow rate at least three times your tank volume per hour — a 20-gallon tank needs a filter rated for 60 gallons per hour or higher. Hang-on-back filters are the cheapest and easiest to maintain. Canister filters cost more but take up less space and are quieter. Sponge filters are silent and good for small tanks but require an air pump.

The heater keeps water temperature stable. Most tropical fish need 76–78 degrees Fahrenheit. Buy a heater rated for your tank size — a 50-watt heater works for a 10-gallon tank, a 100-watt for a 20-gallon, and a 150-watt for a 40-gallon. Submersible heaters sit inside the tank; hang-on heaters clip to the rim. A thermometer lets you check the temperature daily. Do not skip the heater even if your room feels warm — temperature swings stress fish and invite disease.

Rinse the gravel and add substrate, decorations, and plants

Gravel traps waste and gives bacteria a place to colonize. Rinse it in a bucket under running water until the water runs clear — this removes dust that clouds the tank. Spread 2 to 3 inches of rinsed gravel across the tank bottom. You can use sand instead, but it compacts and needs stirring during maintenance.

Add decorations, plants, and hiding spots next. Live plants consume nitrate and produce oxygen, which speeds the cycling process. Java fern, anubias, and moss are hardy and don't need special lighting. Plastic plants work too but do nothing for water quality. Driftwood, rocks, and caves give fish shelter and reduce stress. Arrange everything before you fill the tank because moving it afterward stirs up sediment and disturbs bacteria colonies.

Fill the tank with treated water and run the filter for one to two weeks

Tap water contains chlorine and chloramine that kill beneficial bacteria. Buy a dechlorinator at any aquarium store and follow the bottle's instructions — most require just a few drops per gallon. Fill a bucket, treat it, and pour it slowly into the tank. This prevents gravel from shifting and keeps decorations in place. Fill the tank to about an inch below the rim.

Turn on the filter and heater. Set the heater to 76–78 degrees and check the thermometer after a few hours to confirm it's working. The filter should run 24 hours a day. This period — called the nitrogen cycle — takes one to two weeks. During this time, ammonia from fish waste (or from a small pinch of fish food if you add none) converts to nitrite, then to nitrate. Beneficial bacteria consume ammonia and nitrite, making the tank safe for fish. You cannot see this happening, but it is essential.

Test the water before adding any fish

Buy a liquid test kit that measures ammonia, nitrite, and nitrate. Do not use test strips — they are less accurate. Test the water every few days during the cycling period. You are waiting for ammonia and nitrite to drop to zero and nitrate to rise above zero. This signals that bacteria are established and the tank is ready.

If ammonia or nitrite stays high after two weeks, wait longer. Some tanks take three to four weeks. If you see no change after three weeks, the tank may not be cycling — this sometimes happens if the water is too cold or if you used chlorinated water without a dechlorinator. Check the heater temperature and make sure you treated the water. If both are correct, do a 25 percent water change, treat the new water, and test again in a few days.

Add fish slowly after the tank cycles

Once ammonia and nitrite are zero, add fish gradually. Start with hardy species like tetras, danios, or corydoras catfish. Add only a few fish at first — no more than one inch of fish per gallon of water. A 20-gallon tank can hold 20 inches of fish, but start with 5 to 10 inches and wait a week before adding more. This prevents ammonia from spiking and crashing the cycle.

Acclimate new fish before releasing them into the tank. Float the bag they came in on the water surface for 15 minutes so the temperature inside matches the tank. Then slowly add tank water to the bag over the next 15 minutes, a cup at a time. This prevents shock. Use a net to transfer the fish into the tank and discard the bag water — it may contain disease or store chemicals.

Maintain the tank with weekly water changes and filter cleaning

Once fish are in, do a 25 percent water change every week. Siphon water from the bottom with a gravel vacuum, which removes waste trapped in the gravel. Treat the new water with dechlorinator before adding it. Clean the filter cartridge or sponge in old tank water — never use tap water, which kills the bacteria living on it. Rinse the cartridge gently; do not scrub it.

Test the water once a week for the first month, then every two weeks after that. Ammonia and nitrite should stay at zero. Nitrate will rise over time and should stay below 40 parts per million. If nitrate climbs above 40, do larger water changes — 30 to 50 percent — or add more plants. Check the heater temperature daily and the filter intake for clogs. Replace the filter cartridge every four to six weeks, or when water flow slows.

Frequently Asked Questions

Can I add fish the same day I set up the tank?

No. Fish produce ammonia when ready, and without established bacteria to consume it, ammonia builds up and poisons them within hours. The nitrogen cycle takes one to two weeks. Rushing this step kills fish. Some people add a small amount of fish food or a pinch of fish waste from an established tank to start the cycle without fish present.

What if the water is cloudy after I fill the tank?

Cloudiness usually means sediment from the gravel or a bacterial bloom from the cycling process. It clears on its own within a few days as the filter traps particles and bacteria settle. Do not do a water change — it resets the cycle. If cloudiness lasts more than a week, check that the filter is running and the intake is not clogged.

Do I need a light on the tank?

A light helps you see the fish and supports live plants if you have them. Most aquarium lights run 8 to 10 hours per day on a timer. Avoid leaving the light on 24 hours — it stresses fish and causes algae blooms. If you use only plastic decorations and no plants, a light is optional but makes the tank easier to enjoy.

What temperature should the water be?

Most tropical fish thrive at 76–78 degrees Fahrenheit. Goldfish and some other coldwater fish do better at 65–72 degrees and do not need a heater if your room stays cool. Check what temperature your specific fish need before buying them, because mixing coldwater and tropical fish in one tank does not work.

How often should I clean the tank glass?

Clean the inside glass weekly with an algae scraper or soft cloth to keep it clear. Algae grows on glass as a thin green film and is harmless, but it blocks your view. Do not use soap or chemicals — just water and a scraper. The outside glass can be cleaned with glass cleaner anytime.