Start with the right tank size and shape for your space

The first decision is how much room you have and what you want to keep in it. A 20-gallon tank is the practical minimum for most freshwater setups — smaller tanks swing in temperature and water chemistry too quickly, and fish waste builds up faster. A 10-gallon tank works only for a few hardy species like bettas or small groups of tetras. If you have the space, 40 gallons or larger gives you more flexibility and is actually easier to maintain because the water stays more stable.

Tank shape matters less than volume, but a longer, shallower tank gives fish more swimming room than a tall, narrow one. Standard rectangular tanks (20-gallon long, 55-gallon, 75-gallon) are cheaper and easier to find than custom shapes, and they work with standard equipment like filters and heaters.

Key Takeaways

  • A 20-gallon tank is the practical starting point; anything smaller requires daily monitoring and frequent water changes.
  • You will need a filter rated for your tank size, a heater set to the species you choose, and an air pump or power head to move water.
  • Substrate (gravel or sand), plants or decorations, and a light source complete the physical setup before you add water.
  • Let the tank run for one to two weeks before adding fish so beneficial bacteria can establish in the filter.
  • Weekly water changes of 20 to 30 percent and regular filter cleaning keep the tank stable and fish healthy.

Choose and install a filter matched to your tank volume

The filter is the most important piece of equipment you will buy. It removes waste, uneaten food, and dead plant matter, and it provides a home for beneficial bacteria that break down fish waste into less toxic compounds. A filter rated for your tank size should process the entire volume at least three times per hour — a 20-gallon tank needs a filter rated for 60 gallons per hour (GPH) or higher.

The three common types are hang-on-back filters (easiest to install, clips to the rim), canister filters (sits under the tank, handles larger setups), and sponge filters (straightforward, quiet, best for small tanks or breeding setups). For a first tank, a hang-on-back filter is usually the right choice — it is affordable, reliable, and you can see when it needs cleaning.

Install the filter before you fill the tank. Most hang-on-back filters come with intake and output tubes; position the intake near the bottom of the tank and the output at the surface so water circulates throughout. Do not turn it on yet — wait until the tank is full.

Add a heater and thermometer to maintain stable temperature

Most tropical fish need water between 76 and 80 degrees Fahrenheit. A submersible heater sits inside the tank and maintains that range automatically. Size it to your tank: a 50-watt heater works for 10 to 20 gallons, a 100-watt for 20 to 40 gallons, and a 150-watt for 40 to 60 gallons. If your room is very cold, go one size larger.

Place the heater near the filter output so warm water circulates evenly. Mount a thermometer on the opposite side of the tank so you can see the temperature at a glance. Stick-on thermometers are cheap and work fine. Do not plug in the heater until the tank is full and has been running for a few minutes — running a heater in air damages it.

Prepare the substrate and add hardscape before filling

Substrate is the material on the bottom of the tank. Gravel is the most common choice — it looks natural, lets water flow through it, and is straightforward to clean. Rinse it thoroughly in a bucket before adding it to the tank, or it will cloud the water. Spread it 2 to 3 inches deep across the bottom. Sand is an alternative if you want plants with roots or if you are keeping bottom-feeding fish like corydoras catfish, but it compacts more easily and requires gentler cleaning.

Before you fill the tank, arrange any rocks, driftwood, or decorations. These pieces anchor plants, create hiding spots for fish, and break up the tank visually. Make sure anything you add is aquarium-safe — some rocks leach minerals that change water chemistry, and some wood is treated with chemicals. Buy hardscape from an aquarium store if you are unsure.

Fill the tank and run it for one to two weeks before adding fish

Fill the tank slowly with dechlorinated water. Tap water contains chlorine that harms fish and kills beneficial bacteria, so either let the water sit for 24 hours before adding it, or use a water conditioner (a few drops neutralize chlorine when ready). Pour water onto a plate or into your hand so it does not blast the substrate and decorations around.

Once the tank is full, turn on the filter and heater. Set the heater to 78 degrees and let the tank run. This period, called cycling, allows beneficial bacteria to colonize the filter media and substrate. You can test the water with an inexpensive kit to watch ammonia and nitrite drop to zero — that signals the tank is ready. If you do not test, two weeks is usually enough for a lightly planted tank.

During this time, you can add live plants if you want them. Plants consume some fish waste and produce oxygen, which helps keep the water clean. straightforward plants for beginners include Java fern, Anubias, and Amazon sword — tie them to rocks or driftwood rather than burying them, or they will rot.

Add lighting and choose fish suited to your setup

Most aquariums come with a basic hood and light. If yours does not, an LED light strip mounted above the tank works well and uses little electricity. Aim for 8 to 10 hours of light per day — more than that encourages algae growth. A straightforward timer plugged into the wall turns the light on and off automatically.

Before you buy fish, research what species work together and what your tank can support. A 20-gallon tank might hold 10 small tetras, or 5 medium fish like guppies, or 2 larger fish like dwarf cichlids — the rule of thumb is 1 inch of fish per gallon, though that varies by species and how much you are willing to maintain. Start with hardy, peaceful species: tetras, guppies, corydoras catfish, or dwarf gouramis. Avoid aggressive fish or species that need more space than you have.

Perform regular maintenance to keep the tank stable

Once fish are in the tank, maintenance keeps them healthy. Every week, remove 20 to 30 percent of the water and replace it with fresh, dechlorinated water. This removes waste that the filter cannot catch and replenishes minerals fish need. Use a gravel vacuum (a tube with a bulb that siphons water while stirring the substrate) to clean the bottom at the same time.

Every two to four weeks, rinse the filter media in old tank water (not tap water, which kills the bacteria). Never replace the media entirely — you want to keep the bacteria colony. If the filter output slows, the media is clogged and needs rinsing. Check the heater monthly to make sure it is still set correctly, and replace it if it stops heating or leaks.

Watch for algae growth on the glass and decorations. A little algae is normal and harmless, but if it covers everything, reduce the light hours or do more frequent water changes. An algae scraper or magnetic cleaner removes it from the glass without chemicals.

Frequently Asked Questions

Can I use tap water straight from the faucet?

Not without treating it first. Tap water contains chlorine and sometimes chloramine, which kill fish and beneficial bacteria. Either let the water sit in an open bucket for 24 hours so chlorine evaporates, or add a water conditioner (a few drops per gallon) and use it when ready. For large water changes, a conditioner is faster.

How long does it take before I can add fish?

One to two weeks is typical. The tank needs time for beneficial bacteria to establish in the filter. You can speed this up by adding a bacterial starter product or by moving some filter media from an established tank, but even then, wait at least a few days before adding fish. Adding too many fish too quickly causes ammonia spikes that kill them.

What should I do if the water turns cloudy?

Cloudiness usually means either the substrate was not rinsed (fine particles suspended in water) or a bacterial bloom is happening (the tank is cycling and bacteria are multiplying). Do not do a large water change — that removes the bacteria you need. Wait a few days; the cloudiness will clear on its own. If it does not, do a 25 percent water change and check that the filter is running.

Can I keep fish in a bowl without a filter?

Not long-term. A bowl has no room for beneficial bacteria to live, and waste builds up too fast. Fish in bowls need daily water changes and die young. A small tank with a filter is cheaper and easier than a bowl, and the fish live much longer.

What is the most common mistake people make when building a tank?

Adding fish before the tank has cycled. New tanks have no beneficial bacteria, so ammonia from fish waste builds up and poisons them. Waiting one to two weeks before adding fish prevents this. The second mistake is overstocking — too many fish in too small a space. Start with fewer fish than you think the tank can hold; you can always add more later.