Start with the right tank size and location

A fish aquarium needs a solid, level surface away from direct sunlight, heating vents, and air conditioning units. Temperature swings and algae blooms from sunlight will stress your fish and make the tank harder to maintain. A 20-gallon tank is a practical starting point—large enough to keep water chemistry stable, small enough to fit in most homes and not overwhelm a beginner.

Before you buy anything, measure the space where the tank will sit. Check that the surface can hold the weight: a 20-gallon tank filled with water, gravel, and decorations weighs roughly 200 pounds. A sturdy table, stand, or cabinet rated for aquariums works best. Avoid placing the tank near a window or in direct sunlight, which causes algae to grow faster than you can manage.

Key Takeaways

  • A 20-gallon tank on a sturdy, level surface away from sunlight is a good starting point for most people.
  • You need a filter, heater (for tropical fish), air pump or airstone, substrate, and a water conditioner before you add any fish.
  • Let the tank run for 7 to 10 days before adding fish so beneficial bacteria can establish in the filter.
  • Test the water with a kit that measures ammonia, nitrite, and nitrate before introducing fish, and perform partial water changes every week or two.
  • Different fish have different temperature and space needs, so research the species you want before you buy them.

Gather the essential equipment

A working aquarium needs five core pieces of equipment. A filter removes waste and debris; a hang-on-back or canister filter works for most home tanks. A heater is essential if you plan to keep tropical fish, which need water between 75 and 80 degrees Fahrenheit. A thermometer lets you monitor temperature. An air pump with an airstone adds oxygen and creates water movement, though some filters do this on their own. Substrate—gravel or sand—covers the bottom and gives beneficial bacteria a place to live.

You will also need a water conditioner to remove chlorine and chloramine from tap water before it touches the tank. A basic aquarium test kit measures ammonia, nitrite, and nitrate levels so you know when the tank is safe for fish. A net, bucket, and siphon make water changes easier. Buy these items before you set up the tank so you have everything ready.

Assemble the tank and run it empty for a week

Place the empty tank on its stand and check that it is level using a spirit level. Uneven tanks can crack or leak. Rinse the gravel or sand in a bucket under running water until the water runs clear—this removes dust and debris. Spread 2 to 3 inches of substrate across the tank bottom.

Fill the tank halfway with tap water treated with water conditioner, then place your filter, heater, and air pump in the tank according to their instructions. Fill the tank the rest of the way. Turn on all equipment and let it run for 7 to 10 days without any fish. This period allows beneficial bacteria to colonize the filter and substrate, creating a stable environment. During this time, test the water daily with your test kit. Ammonia and nitrite should be zero or very close to it before you add fish.

Test water chemistry before adding fish

Use your aquarium test kit to measure ammonia, nitrite, and nitrate. Ammonia and nitrite should both read zero or near zero. Nitrate may be slightly elevated, which is normal. If ammonia or nitrite are high after 10 days, wait another few days and test again. Some tanks take longer to cycle, especially if the water is cold.

Once ammonia and nitrite are stable at zero, the tank is ready for fish. Add fish slowly—introduce only a few at first, then wait a week before adding more. This prevents ammonia spikes that can kill fish. A good rule is one inch of fish per gallon of water, though this varies by species. A 20-gallon tank can safely hold about 20 inches of fish total, but start with less.

Choose fish suited to your tank size and temperature

Different fish have different needs. Goldfish need cooler water (around 65 to 72 degrees) and grow large, so they need bigger tanks than most people realize. Tropical fish like tetras, guppies, and corydoras catfish thrive in warm water (75 to 80 degrees) and do well in 20-gallon tanks. Bettas are solitary and aggressive toward other males but can live in smaller tanks if the water is warm and the filter is gentle.

Before you buy any fish, research the species online or ask at an aquarium store. Find out how large they grow, what temperature they need, whether they are aggressive or peaceful, and how many of their kind can live together. Mixing incompatible fish leads to stress, disease, and death. Start with hardy, peaceful species like neon tetras, guppies, or corydoras catfish if you are new to fishkeeping.

Perform regular water changes and monitor the tank

Once fish are in the tank, test the water weekly for the first month, then every two weeks after that. Watch for ammonia or nitrite spikes, which mean the tank is becoming overloaded. If either reading rises above zero, do a partial water change when ready—remove 25 to 30 percent of the water and replace it with treated tap water at the same temperature.

Perform a partial water change every one to two weeks as part of routine maintenance, even if test results look good. This removes nitrate buildup and keeps the water fresh. Never do a complete water change unless the tank is empty or you are treating a serious problem; removing all the water kills the beneficial bacteria. Clean the filter according to its instructions, usually by rinsing the filter media in old tank water (not tap water, which kills bacteria). Feed fish once or twice daily, only as much as they can eat in a few minutes—uneaten food decays and fouls the water.

Troubleshoot common problems

Cloudy water in a new tank usually means the bacterial colony is still establishing. It clears on its own within a few days. If the water stays cloudy after two weeks, do a partial water change and check that you are not overfeeding. A strong smell or visible algae growth means the tank is not being maintained—increase water change frequency and reduce feeding.

Fish gasping at the surface or hiding constantly may indicate low oxygen or high ammonia. Check the heater and thermometer to make sure the temperature is correct. Make sure the filter and air pump are running. Test the water for ammonia and nitrite. If ammonia is high, do a partial water change right away. If fish are dying despite normal water chemistry, the problem may be disease or incompatible tank mates. Isolate sick fish in a separate container and research the symptoms, or ask for help at a local aquarium store.

Frequently Asked Questions

How long does it take to set up a tank before fish can go in?

Most tanks are ready in 7 to 10 days if you let the filter run continuously and test the water daily. Some tanks take longer, especially if the water is cold. Do not rush this step—adding fish to an uncycled tank causes ammonia spikes that kill them.

Can I use tap water straight from the faucet?

No. Tap water contains chlorine and chloramine, which are toxic to fish. Always treat tap water with a water conditioner before it enters the tank. The conditioner neutralizes these chemicals when ready.

What should I feed my fish?

Most aquarium fish do well on commercial fish flakes or pellets made for their species. Feed once or twice daily, only as much as they can eat in a few minutes. Uneaten food sinks and decays, raising ammonia levels. Overfeeding is the most common mistake new aquarium keepers make.

Do I need a heater if I keep goldfish?

No. Goldfish prefer cool water between 65 and 72 degrees and do not need a heater in most homes. Tropical fish like tetras and guppies do need a heater to stay healthy.

What does it mean if my fish are hiding all the time?

Hiding can mean the water chemistry is wrong, the temperature is off, the tank is too small, or the fish are stressed by aggressive tank mates. Test the water for ammonia and nitrite, check the thermometer, and research whether the fish you chose are compatible with each other.