Start with the right tank size and shape for your space

The first decision is the tank itself. A 20-gallon tank is the practical minimum for a beginner—smaller tanks swing in temperature and water chemistry too quickly, and larger tanks are more forgiving of mistakes. A standard 20-gallon is 24 inches long, 12 inches wide, and 16 inches tall. If you have less floor space, a 10-gallon works, but you'll have fewer fish options and less room for error.

Glass tanks are standard and affordable. Acrylic tanks are lighter and won't shatter, but they scratch easily and cost more. The shape matters less than the surface area—a longer, shallower tank gives fish more room to swim than a tall, narrow one, and it holds more oxygen at the water's surface. Avoid decorative bowls and hexagon tanks; they don't hold enough water to stay stable.

Place the tank on a sturdy, level stand before you fill it. A filled 20-gallon tank weighs about 170 pounds. The stand should be rated for the weight, and the floor underneath should be solid—not carpet directly under the legs, which can rot and cause the stand to sink unevenly.

Key Takeaways

  • A 20-gallon glass tank on a rated stand is the practical starting point for most home aquariums.
  • A filter rated for your tank size, an air pump or power head, and a heater (for tropical fish) are the three systems you must have working before adding fish.
  • Substrate, rocks, and plants go in before water; the tank cycles for one to two weeks before fish are added, so bacteria can establish.
  • Water changes of 25 percent every two weeks keep nitrate levels down and fish healthy.
  • Test your water weekly for ammonia, nitrite, and nitrate until the tank is stable, then monthly after that.

Install filtration, aeration, and heating before adding water

The filter is the most important piece of equipment. It removes waste and gives beneficial bacteria a place to live. A hang-on-back filter (also called a power filter) is the easiest for beginners—it clips to the rim and draws water through a cartridge. Buy one rated for your tank size or slightly larger; a filter rated for 30 gallons works well on a 20-gallon tank. Replace the cartridge every month, or rinse it in old tank water if the manufacturer allows it.

An air pump or power head adds oxygen and water movement. Many hang-on filters do this already, but if your filter is small or your tank is crowded, add an air stone (a porous stone on the end of a tube) or a small power head. This is especially important if you keep fish that need high oxygen, like goldfish.

A heater is necessary only if you keep tropical fish (most colorful aquarium fish). A 50-watt heater works for a 10- to 20-gallon tank. Set it to 76 to 78 degrees Fahrenheit. Goldfish and some other species prefer cooler water and don't need a heater. Install the heater, filter, and air pump, then fill the tank halfway and run them for a few hours to check for leaks before you add substrate.

Add substrate, hardscape, and plants in the right order

Substrate is the material on the bottom—usually gravel or sand. Rinse gravel thoroughly under running water until the water runs clear; it holds dust and debris that will cloud your tank. Spread 1 to 2 inches across the bottom. Sand is finer and looks more natural, but it compacts and can trap waste; if you use sand, stir it gently during water changes.

Rocks, driftwood, and other hardscape come next. Arrange them before you fill the tank completely, so you can see what you're doing and avoid crushing plants or creating unstable stacks. Rocks should not be limestone or marble unless you want to raise the pH and hardness of your water; slate, granite, and lava rock are safe choices. Driftwood lowers pH slightly and is fine for most fish.

Live plants are optional but helpful—they consume nitrate (a waste product fish produce) and give fish places to hide. straightforward beginner plants include Java fern, Anubias, and Amazon sword. Plant them in the substrate or tie them to rocks with fishing line; they'll anchor themselves over time. Artificial plants work too and require no light or fertilizer, but they don't reduce nitrate.

Fill the tank and let it cycle for one to two weeks

Fill the tank slowly with dechlorinated water. Tap water contains chlorine, which kills beneficial bacteria. Use a water conditioner (sold at any aquarium store) to remove it, or let the water sit in an open bucket for 24 hours and the chlorine will evaporate. Pour the water over a plate or your hand to avoid disturbing the substrate.

Turn on the filter, heater, and air pump. The tank will look cloudy for a few days—this is normal and is usually a bacterial bloom that will clear on its own. Do not add fish yet. The tank is now cycling, which means beneficial bacteria are colonizing the filter and substrate. These bacteria convert ammonia (from fish waste) into nitrite, then into nitrate, which is less toxic.

Cycling takes one to two weeks. You can speed it up by adding a bacterial starter culture (sold as "beneficial bacteria" or "cycle starter"), but it's not required. Test the water every few days with a liquid test kit (not the strips—they're less accurate). You're looking for ammonia and nitrite to spike, then drop to zero, and nitrate to appear. When ammonia and nitrite are both zero and nitrate is present, the tank is cycled and ready for fish.

Choose fish that match your tank size and water conditions

A 20-gallon tank can hold roughly 20 gallons of fish—meaning one inch of fish per gallon, though this is a rough guide. A 2-inch fish counts as 2 gallons. Goldfish are the exception; they need 20 gallons per fish, so one goldfish per 20-gallon tank.

Beginner fish that do well in a 20-gallon tank include tetras (small, colorful, peaceful), guppies (hardy, colorful, breed easily), corydoras catfish (bottom feeders, peaceful), and platies (hardy, colorful, peaceful). Avoid large or aggressive fish like cichlids or plecos until you have experience. Check the temperature and pH requirements of any fish before you buy it; most tropical fish want 76 to 78 degrees and a pH between 6.5 and 7.5.

Add fish slowly—no more than a few at a time, spaced a few days apart. This gives the bacteria time to adjust to the extra waste. Float the bag the fish came in on the surface of the tank for 15 minutes so the temperature equalizes, then open the bag and let the fish swim out. Do not pour the bag water into the tank; it may carry disease.

Perform water changes and test regularly to keep the tank stable

Change 25 percent of the water every two weeks. Use a siphon (a tube that uses gravity to drain water) or a small bucket. Remove the water from the bottom, where waste settles, and replace it with fresh dechlorinated water at the same temperature. This removes nitrate, which builds up over time and stresses fish.

Test the water weekly for the first month, then every two weeks after that. Use a liquid test kit that measures ammonia, nitrite, nitrate, and pH. If ammonia or nitrite spike above zero, you've added too many fish or the filter is clogged—do a 25 percent water change and clean the filter cartridge. Nitrate should stay below 40 parts per million; if it's higher, increase water changes to 25 percent weekly.

Clean the filter cartridge monthly by rinsing it in old tank water (not tap water, which kills the bacteria). Replace it every three months or when water flow slows. Never clean the substrate, rocks, or plants with soap; rinse them in old tank water only.

Troubleshoot common problems before they harm your fish

Cloudy water in the first week is usually a bacterial bloom and clears on its own. If it persists after two weeks, the filter may be undersized or clogged. Clean the cartridge and check that water is flowing freely.

Fish gasping at the surface means low oxygen. Check that the air pump or filter is running, add an air stone if you don't have one, and do a 25 percent water change. If the problem continues, the tank may be overstocked or the heater may be set too high (warm water holds less oxygen).

Dead fish or fish hiding constantly suggests ammonia or nitrite is high. Test the water when ready. If ammonia or nitrite is above zero, do a 50 percent water change, clean the filter, and reduce feeding until the levels drop. If you see white spots on the fish or torn fins, the tank may have a disease—isolate the sick fish in a separate small tank and treat it with medication from an aquarium store.

Algae growth is normal and usually harmless. Green algae on the glass is a sign the tank is getting enough light. Wipe it off with a magnetic scraper. If brown algae (diatoms) or black algae appear, reduce light to 8 hours a day and increase water changes.

Frequently Asked Questions

How long does it take to set up a tank before I can add fish?

One to two weeks. You need to fill the tank, run the filter and heater, and let beneficial bacteria establish. You can speed this up with a bacterial starter culture, but you still need to test the water and confirm ammonia and nitrite are zero before adding fish.

Can I use tap water straight from the faucet?

No. Tap water contains chlorine, which kills beneficial bacteria and harms fish. Use a water conditioner to remove it, or let the water sit in an open bucket for 24 hours. Some tap water also contains ammonia or heavy metals; if your fish die for no clear reason, have your water tested at a local aquarium store.

What should I feed my fish and how often?

Feed once or twice a day, only as much as the fish can eat in two to three minutes. Uneaten food rots and raises ammonia. Most flake or pellet food is fine; follow the package instructions. Overfeeding is the most common beginner mistake and causes ammonia spikes.

Do I need a light for my aquarium?

Not if you don't have live plants. If you do, 8 to 10 hours of light per day is enough. A basic LED light strip is affordable and uses little electricity. Avoid direct sunlight, which causes algae blooms.

What do I do if I have to leave for a week?

Fish can go a week without food. Do a 25 percent water change before you leave, and don't feed them the day you go. A battery-powered air pump is a good backup in case the power goes out. Do not use automatic feeders; they often overfeed and foul the water.