Start with a tank, filter, and heater—the three things that keep fish alive

A working aquarium needs three things: a tank to hold water, a filter to remove waste, and a heater to keep the temperature steady. Everything else—plants, decorations, lighting—comes after those three are in place and running. Most beginners buy a kit that includes the tank, filter, heater, and light together, which costs less than buying each piece separately and ensures the filter is sized correctly for the tank volume.

The tank itself should be at least 20 gallons if you plan to keep more than a few fish. Smaller tanks swing in temperature and chemistry faster, which stresses fish and makes them sick. A 20-gallon tank is roughly 24 inches long, 12 inches wide, and 16 inches tall. Larger tanks are more forgiving and easier to maintain, but they cost more and take up more space. Glass tanks last longer than plastic and don't scratch as easily, but they are heavier and break if dropped.

Key Takeaways

  • A functioning aquarium requires a tank, filter, and heater; everything else is optional but improves fish health and appearance.
  • Start with at least 20 gallons so water temperature and chemistry stay stable enough for fish to survive.
  • Run the filter and heater for one to two weeks before adding fish so bacteria can colonize the filter and establish the nitrogen cycle.
  • Test the water for ammonia, nitrite, and nitrate before adding fish, and perform a 25 percent water change every week to remove waste.
  • Choose fish species that tolerate the same temperature and water chemistry, and research their adult size before buying them.

Choose a filter sized for your tank volume

The filter removes fish waste and uneaten food before they poison the water. A hang-on-back filter clips to the rim of the tank and is the cheapest and easiest to install. A canister filter sits under the tank and handles larger volumes, but costs more and requires more plumbing. A sponge filter is silent and works well for small tanks or breeding setups, but moves water slowly.

The filter's flow rate should be at least three times the tank volume per hour. For a 20-gallon tank, you need a filter rated for 60 gallons per hour or higher. Check the filter's packaging or manual for its flow rate before you buy. A filter that is too weak will not remove waste fast enough; a filter that is too strong will create too much current and stress the fish.

The filter contains media—sponges, ceramic rings, or charcoal—that traps waste and grows beneficial bacteria. When you first set up the tank, the media is sterile and contains no bacteria. You must run the filter for one to two weeks before adding fish so bacteria can grow and break down ammonia into less toxic compounds. This process is called cycling the tank.

Install a heater and thermometer to maintain stable temperature

Most tropical fish need water between 75 and 80 degrees Fahrenheit. A submersible heater sits inside the tank and heats the water directly. A hang-on heater clips to the rim and heats water as it flows past. Submersible heaters are cheaper and more common. Buy a heater rated for your tank size—a 50-watt heater works for 10 to 20 gallons, a 100-watt heater for 20 to 40 gallons, and a 200-watt heater for 40 to 60 gallons.

Place a thermometer on the opposite side of the tank from the heater so you can see the actual water temperature. Stick-on thermometers are cheap but less accurate than floating or digital thermometers. Check the temperature every morning and evening for the first week to make sure the heater is holding the target temperature. If the water is too cold or too hot, adjust the heater dial or replace it with a different wattage.

Prepare the substrate and add water

The substrate is the material on the bottom of the tank. Gravel is the most common choice—it looks natural, lets beneficial bacteria grow, and makes it straightforward to remove waste with a siphon. Sand looks more natural but compacts over time and can trap waste. Bare bottom (no substrate) is easiest to clean but looks plain and gives bacteria less surface area to colonize.

Rinse the gravel or sand in a bucket under running water until the water runs clear. This removes dust and dirt that will cloud the tank. Spread the rinsed substrate across the bottom of the tank in a layer about 2 inches deep. Add decorations—rocks, driftwood, or plastic plants—to give fish places to hide and rest.

Fill the tank slowly with tap water. If your tap water is heavily chlorinated, let it sit for 24 hours before filling the tank, or use a water conditioner to remove chlorine when ready. Pour water onto a plate or into your hand so it does not blast the substrate and decorations out of place. Fill the tank until the water level is about 1 inch below the rim.

Run the filter and heater for one to two weeks before adding fish

Turn on the filter and heater and let them run continuously. The heater will bring the water to the target temperature within a few hours. The filter will begin to move water through the media, but no beneficial bacteria exist yet. Over the next one to two weeks, bacteria from the air and from the tap water will land on the filter media and begin to multiply. This is the nitrogen cycle—bacteria convert ammonia (from fish waste) into nitrite, then into nitrate, which is less toxic.

You can speed up cycling by adding a bacterial starter—a bottle of live bacteria that you pour into the tank. These products cost $10 to $20 and can cut cycling time from two weeks to a few days. Without a starter, cycling takes longer but happens on its own.

After one week, test the water with a test kit that measures ammonia, nitrite, and nitrate. Buy a liquid test kit, not strips—strips are less accurate. The ammonia and nitrite should be zero, and nitrate should be above zero. If ammonia or nitrite is still high after two weeks, wait another week before adding fish.

Choose fish that match your tank size and water conditions

Research each species before you buy. Some fish need warm water, others need cool water. Some grow to 12 inches, others stay under 2 inches. Some are aggressive and will eat smaller fish, others are peaceful. A betta is a popular choice for small tanks because it tolerates cool water and needs little space, but males will fight each other. Tetras and guppies are peaceful, colorful, and work well in 20-gallon tanks. Corydoras catfish eat waste off the bottom and are hardy.

A common rule is one inch of fish per gallon of water. A 20-gallon tank can hold 20 inches of fish total—for example, ten 2-inch tetras or five 4-inch guppies. This rule is rough and varies by species, so research the adult size of each fish you want to keep.

Buy fish from a store where the tanks are clean and the fish look healthy—no torn fins, no spots, no gasping at the surface. Ask the store worker how long the fish have been in stock; newly arrived fish are more stressed and more likely to get sick.

Add fish slowly and monitor water chemistry weekly

Add only a few fish at a time—no more than one-third of the total number you plan to keep. Wait one week, then add more. This lets the bacteria in the filter grow to match the amount of waste the fish produce. If you add all the fish at once, ammonia and nitrite will spike and poison them.

After each addition, test the water for ammonia and nitrite. Both should stay at zero. If either rises above 0.5 parts per million, do a 25 percent water change—remove one-quarter of the water and replace it with fresh, dechlorinated water. This dilutes the waste and buys time for the bacteria to catch up.

Once the tank is fully stocked, test the water once a week. Perform a 25 percent water change every week to remove nitrate, which builds up over time and stresses fish. Clean the filter media once a month by rinsing it in old tank water (not tap water, which kills the bacteria). Replace the filter media only when it falls apart or stops moving water.

Maintain the tank with weekly water changes and monthly filter cleaning

An aquarium is not a closed system—fish produce waste, plants die, and chemicals build up. You must remove waste regularly or the water becomes toxic. A 25 percent water change every week is the standard maintenance routine for most tanks. Use a siphon (a tube that uses gravity to drain water) to remove water from the bottom of the tank where waste settles. This also removes uneaten food and dead plant matter.

Feed fish only as much as they can eat in two to three minutes, once or twice a day. Uneaten food rots and pollutes the water. Most fish are hungry all the time and will beg for food, but overfeeding is the most common mistake beginners make.

Check the heater and thermometer every day. If the water temperature drops more than a few degrees, the heater may be broken and needs replacement. If algae covers the glass, wipe it with a soft cloth or an algae scraper. If the filter stops moving water, the media is clogged and needs cleaning.

Frequently Asked Questions

How long does it take to set up an aquarium before I can add fish?

One to two weeks. The filter and heater must run continuously during this time so bacteria can colonize the filter media and establish the nitrogen cycle. You can add fish sooner if you use a bacterial starter product, which can reduce cycling time to a few days. Test the water before adding fish to confirm ammonia and nitrite are zero.

What is the cheapest way to start an aquarium?

Buy a 20-gallon kit that includes the tank, filter, heater, and light. Kits cost $100 to $200 and are cheaper than buying each piece separately. Add gravel, a thermometer, and a test kit. Avoid buying many decorations or plants at first—focus on getting the water chemistry stable, then add extras later.

Can I use tap water straight from the faucet?

Yes, if your tap water is not heavily chlorinated. If your water smells like a swimming pool, use a water conditioner to remove chlorine before filling the tank. Let tap water sit for 24 hours and most chlorine will evaporate on its own. Never use distilled water or rainwater alone—fish need minerals that are in tap water.

What should I do if my fish are gasping at the surface?

This usually means ammonia or nitrite is too high, or the water is too warm and does not hold enough oxygen. Test the water when ready. If ammonia or nitrite is above zero, do a 25 percent water change right away. Check that the filter is running and moving water. If the water temperature is above 82 degrees, lower the heater setting or increase air circulation with a fan.

How often should I replace the filter media?

Only when it falls apart or stops moving water—usually after six months to a year. Rinsing the media in old tank water once a month keeps it clean without killing the bacteria. Never replace the media with new media all at once, or you will lose the bacteria and the tank will cycle again. If the media is very dirty, replace half of it and keep the other half to preserve bacteria.