Start with the right tank size and location

The first decision is how large your aquarium will be. Smaller tanks—10 to 20 gallons—are harder to maintain than larger ones because water chemistry changes faster when the volume is low. A 20-gallon tank or larger gives you more room for error and more stable conditions for fish. Measure your space before you buy: the tank needs a sturdy, level surface that won't flex, and it should sit away from direct sunlight, heating vents, and air conditioning units, all of which destabilize water temperature.

Once you have chosen the location, place the tank on a stand or table rated to hold its weight when full. A 20-gallon tank filled with water weighs roughly 170 pounds. Check that the surface is truly level using a spirit level—an uneven tank can crack over time. Leave space behind and to the side of the tank for filter hoses, heater cords, and your hands during maintenance.

Key Takeaways

  • A tank of 20 gallons or larger is easier to maintain than a small one because the water chemistry stays more stable.
  • The tank must sit on a level, sturdy surface away from direct sunlight and temperature swings from vents or air conditioning.
  • You will need a filter, heater, air pump or airstone, substrate, and a water conditioner before you add any fish.
  • Tap water contains chlorine and chloramine that harm fish, so treat it with a water conditioner before the tank is stocked.
  • The nitrogen cycle takes two to four weeks to establish, so do not add all your fish at once or they will die from ammonia buildup.

Choose and install a filter rated for your tank size

The filter is the most important piece of equipment you will buy. It removes waste and keeps the water safe for fish to breathe. Filters are rated by gallons per hour (GPH)—the amount of water they move through the filter media each hour. For a 20-gallon tank, choose a filter rated for at least 80 to 100 GPH, which means the entire tank volume passes through the filter four to five times per hour.

The most common type for beginners is a hang-on-back filter, which clips to the rim of the tank and draws water up a tube, passes it through filter media (usually foam, charcoal, and biological material), and returns it to the tank. Install it before you fill the tank. Place the intake tube so it draws water from the middle of the tank, not from the surface or the bottom. The outflow should point across the tank to create gentle circulation.

An alternative is a canister filter, which sits under the tank and connects with hoses. Canister filters hold more media and are quieter, but they cost more and require more maintenance. For a first aquarium, a hang-on-back filter is simpler and sufficient.

Add substrate, heater, and aeration

Before you fill the tank, add substrate—the material on the bottom. Sand and gravel both work; sand looks more natural and is easier to clean, while gravel is cheaper and allows water to flow through it more easily. Rinse the substrate thoroughly under running water until the water runs clear, then spread it 1 to 2 inches deep across the tank bottom. This gives beneficial bacteria a place to live and makes the tank look finished.

Install a submersible heater rated for your tank size. A 50-watt heater works for tanks up to 20 gallons; larger tanks need 75 to 100 watts. Place it near the filter intake so heated water circulates throughout the tank. Most heaters have a dial to set the target temperature—for tropical fish, set it to 76 to 78 degrees Fahrenheit. The heater will turn on and off automatically to maintain that temperature.

Add an air pump and airstone if you want visible bubbles and extra oxygen. An air pump is not strictly necessary if your filter creates surface movement (which it should), but it helps, especially in tanks with many fish. Connect the air pump to an airstone—a small porous rock that breaks the air into fine bubbles—and place it in a corner of the tank.

Fill the tank and treat the water

Fill the tank slowly with tap water. Tap water contains chlorine and chloramine, chemicals that kill fish and the beneficial bacteria your tank needs. Before you add any fish, treat the water with a water conditioner. Common brands include Seachem Prime and API Stress Coat. Follow the dosage on the bottle—usually a few drops per gallon—and let the treated water sit for 15 minutes before adding fish.

Turn on the filter and heater. The water will be cloudy at first as particles settle and bacteria begin to colonize the filter media. This is normal. Let the tank run for at least 24 hours before you add any fish. Check that the heater is warming the water to your target temperature and that the filter is running quietly without leaks.

Establish the nitrogen cycle before stocking fish

A new tank is toxic to fish because it lacks the beneficial bacteria that convert ammonia (from fish waste) into nitrite, and then into nitrate (which is less harmful). This process is called the nitrogen cycle, and it takes two to four weeks to establish. If you add all your fish at once, ammonia will spike and kill them.

The safest way to cycle a tank is to add a small number of hardy fish—such as danios or corydoras catfish—and let them produce ammonia while bacteria colonize the filter. Test the water every few days using a liquid test kit (not strips, which are less accurate). You are looking for ammonia to rise, then fall, then nitrite to rise and fall, and finally nitrate to appear. Once ammonia and nitrite are both zero and nitrate is present, the cycle is complete and you can add more fish slowly.

An alternative is to use a bacterial starter culture like Seachem Stability or Dr. Tim's One and Only, which introduces beneficial bacteria directly. Follow the product instructions and still test the water to confirm the cycle has established before adding fish.

Add plants and decorations (optional but helpful)

Live plants are not required, but they absorb nitrate, produce oxygen, and give fish places to hide and explore. straightforward plants for beginners include Java fern, Anubias, and Amazon sword. Plant them in the substrate or tie them to rocks with fishing line. Plastic plants work too if you prefer, but they do not absorb waste.

Add rocks, driftwood, or caves for fish to shelter in. These decorations make the tank look more natural and reduce stress on the fish. Rinse everything under running water before you place it in the tank. Avoid anything sharp that could tear fish fins.

Stock the tank gradually and maintain it weekly

Once the nitrogen cycle is established, add fish slowly—no more than a few at a time, spaced a week apart. This gives the bacteria time to adjust to the increased ammonia load. Research the fish you want before you buy them; some are aggressive, some need groups, and some grow much larger than they appear in the store.

Weekly maintenance keeps the tank healthy. Change 25 percent of the water every week using treated tap water at the same temperature as the tank. Clean the filter intake tube if it clogs. Feed fish once or twice a day, only as much as they can eat in a few minutes—uneaten food rots and fouls the water. Test ammonia, nitrite, and nitrate every two weeks until you are confident the cycle is stable, then monthly after that.

Frequently Asked Questions

Can I use tap water straight from the faucet?

No. Tap water contains chlorine and chloramine that kill fish and beneficial bacteria. Always treat it with a water conditioner before adding it to the tank. A few drops per gallon is all you need, and it takes effect in minutes.

How long before I can add fish after setting up the tank?

At minimum, wait 24 hours so the filter and heater can run and the water can settle. Ideally, wait two to four weeks so the nitrogen cycle can establish. If you add fish before the cycle is complete, ammonia will build up and poison them.

What if my filter is too loud?

Check that the intake tube is fully submerged and that the filter is level. Air in the intake line causes noise. If the filter is old, the impeller may be worn—replace it with a new one from the manufacturer. A canister filter is quieter than a hang-on-back if noise is a persistent problem.

Do I need a heater if the room is warm?

Most tropical fish need stable temperatures between 76 and 78 degrees. If your room temperature drops below 72 degrees at night or in winter, a heater is necessary. Even if your room stays warm, a heater prevents temperature swings that stress fish.

What should I do if the water turns cloudy?

Cloudiness in a new tank is usually bacteria blooming as they colonize the filter—it clears on its own in a few days. Do not do a large water change, which removes the bacteria. If cloudiness persists after a week, test the water for ammonia and nitrite. High levels mean the cycle has not finished or the tank is overstocked.