Start with a tank, filter, and heater—the three things your fish cannot live without

Building an aquarium means assembling a container, water, equipment, and living things into a system that keeps fish alive. You will need a tank (glass or acrylic), a filter to clean the water, a heater to hold temperature steady, and a way to add oxygen. Everything else—plants, decorations, lighting—supports those four basics. Most beginners buy a kit that bundles tank, filter, heater, and light together, which costs less than buying each piece separately and ensures the parts fit.

The tank size matters more than beginners think. A 20-gallon tank is the smallest size that forgives mistakes; smaller tanks swing in temperature and chemistry too fast. A 10-gallon tank works if you commit to weekly water changes and watch it closely. Anything under 5 gallons is difficult for a first build and stresses the fish.

Key Takeaways

  • A working aquarium needs a tank, filter, heater, and air source—buy a kit if you are starting out, because the parts are matched and cheaper together.
  • Fill the tank with dechlorinated water, add substrate (gravel or sand), and run the filter for one to two weeks before adding any fish.
  • The filter builds beneficial bacteria during this "cycling" period, which breaks down fish waste so it does not poison the water.
  • Test the water with a kit before adding fish, and do a 25 percent water change every week to keep ammonia and nitrite at zero.
  • Start with hardy fish like tetras or danios, not fancy varieties, because they survive mistakes while you learn.

Choose a tank size and location before you buy anything

Measure the space where you want the aquarium and make sure the surface can hold the weight. A 20-gallon tank filled with water, gravel, and equipment weighs about 200 pounds. A 55-gallon tank weighs 500 pounds. Check that your shelf, table, or stand is rated for that load and will not sag. Place the tank away from direct sunlight, heating vents, and air conditioners—these cause temperature swings that stress fish and feed algae blooms.

A location near an outlet matters because you will plug in the filter, heater, and light. Keep the tank away from kitchen splashes and pet traffic. Once you fill the tank, moving it is nearly impossible, so choose the spot carefully. If you are renting, check your lease for restrictions on aquariums or water damage liability.

Assemble the tank, filter, heater, and substrate

Rinse the tank inside and out with plain water—no soap. Set it on the stand and check that it sits level using a spirit level. An unlevel tank stresses the seams and can leak. Place the substrate (gravel or sand) in the bottom. Rinse it first by running water through it in a bucket until the water runs clear; this removes dust that clouds the tank. Spread 2 to 3 inches of substrate across the bottom.

Install the heater and filter according to the kit instructions. Most heaters are submersible and clip to the inside wall. Position the heater where water flows past it—usually near the filter intake. Place the filter intake tube away from the heater outlet so heated water does not recirculate without cooling. Fill the tank slowly with dechlorinated water (tap water treated with a dechlorinator product, available at any pet store for a few dollars). Pour water onto a plate or into your hand to slow it and avoid stirring up the substrate.

Turn on the filter and heater. Set the heater to 76 to 78 degrees Fahrenheit for most tropical fish. Let the system run for 24 hours before you do anything else. Check that water is flowing, the heater is warming the water, and there are no leaks.

Cycle the tank for one to two weeks before adding fish

Cycling means running an empty tank so that beneficial bacteria colonize the filter and substrate. These bacteria convert ammonia (from fish waste) into nitrite, then into nitrate, which is far less toxic. Without cycling, ammonia and nitrite spike when you add fish, and the fish die within days. This is the most common beginner mistake.

During cycling, the tank looks empty but the filter is working. You can add a few flakes of fish food every other day to feed the bacteria, or buy a bottled bacteria starter (sold as "beneficial bacteria" or "tank starter"). Let the tank run for one to two weeks. Some people add a hardy fish like a pleco to speed cycling, but this stresses the fish; it is better to wait.

After one week, buy a water test kit (liquid test kits are more accurate than strips). Test for ammonia, nitrite, and nitrate. When ammonia and nitrite both read zero and nitrate reads above 5, the tank is cycled and ready for fish. This usually takes 10 to 14 days. If ammonia or nitrite is still high, wait another week and test again.

Add fish slowly and watch the water chemistry

Start with just two or three small, hardy fish—tetras, danios, or corydoras catfish. These species tolerate beginner mistakes. Avoid fancy goldfish, bettas, or cichlids until you have kept fish for at least six months. Acclimate new fish by floating the bag in the tank for 15 minutes so the temperature inside the bag matches the tank temperature, then slowly add tank water to the bag over 10 minutes. This prevents shock. Use a net to move the fish into the tank; do not pour the bag water into the tank.

Wait one week before adding more fish. Test the water again. If ammonia or nitrite is above zero, do a 25 percent water change (remove one-quarter of the water and replace it with fresh dechlorinated water) and wait another week. Add fish only when ammonia and nitrite stay at zero after a full week. This slow approach keeps the bacteria colony in balance with the fish load.

Perform weekly water changes and monitor the tank

Every week, remove 25 percent of the water and replace it with fresh dechlorinated water. This removes nitrate buildup and keeps the water clean. Use a siphon or gravel vacuum to pull water from the bottom while you work; this also removes uneaten food and waste trapped in the substrate. Never remove more than 30 percent of the water at once, because you will remove too many beneficial bacteria.

Test the water every two weeks for the first two months, then once a month after that. Keep ammonia and nitrite at zero. Nitrate should stay below 40 parts per million. If nitrate climbs above 40, increase water changes to 30 or 40 percent per week. Feed fish once a day, only as much as they eat in two minutes. Uneaten food rots and fouls the water.

Check the heater daily to make sure the temperature stays between 76 and 78 degrees. If the temperature swings more than 2 degrees, the heater may be failing. Clean the filter intake tube if water flow slows; a clogged intake starves the bacteria colony. Most filters have a sponge or cartridge that you rinse in old tank water (never tap water, which kills the bacteria). Replace the cartridge only when rinsing no longer restores flow.

Avoid these common mistakes that kill fish

The biggest mistake is adding fish before the tank is cycled. Ammonia spikes, the fish gasp at the surface, and they die within three to five days. Do not skip the cycling step even if it feels slow. The second mistake is overfeeding. Fish do not need much food; they eat what they can find in the wild. Excess food rots and poisons the water. Feed once a day, a pinch at a time.

The third mistake is doing a large water change all at once. Removing 50 or 75 percent of the water shocks the fish and kills the bacteria. Change 25 percent weekly instead. The fourth mistake is cleaning the filter in tap water. Chlorine in tap water kills the beneficial bacteria you spent weeks growing. Always rinse the filter in old tank water or dechlorinated water.

The fifth mistake is adding too many fish at once. Even a cycled tank can only handle a few new fish per week. Add one or two, wait a week, test the water, then add more. This keeps ammonia and nitrite from spiking. The sixth mistake is using tap water without dechlorinating it. Chlorine burns fish gills and kills bacteria. Buy a dechlorinator bottle (a few dollars) and use it every time you add water.

Frequently Asked Questions

How long does it take to set up an aquarium?

Assembly takes one to two hours. Cycling takes one to two weeks. You cannot add fish until cycling is complete, so plan for at least three weeks from purchase to your first fish. Rushing this timeline is the main reason beginner tanks fail.

Can I use tap water straight from the faucet?

No. Tap water contains chlorine and sometimes chloramine, which burn fish gills and kill beneficial bacteria. Buy a dechlorinator product (a few dollars at any pet store) and treat the water before adding it to the tank. Some dechlorinators also bind heavy metals.

What if my fish are gasping at the surface?

This means the water is low in oxygen or high in ammonia. Check that the filter is running and water is flowing. Make sure the heater is not set too high; warm water holds less oxygen. Do a 25 percent water change when ready. If gasping continues, test the water for ammonia and nitrite. If either is above zero, do another 25 percent change and test again in a few hours.

Do I need plants in my aquarium?

Plants are optional. They look nice and consume some nitrate, but they are not required for the tank to work. Live plants need light and sometimes fertilizer. Plastic plants do the same job for looks without the extra care. Start with plastic plants or no plants at all while you learn the basics.

How often should I clean the tank glass?

Clean the inside glass once a week with an algae scraper or soft cloth. Algae grows on glass and reduces light. Do not use soap or chemicals—plain water and a scraper are enough. Algae is normal and means the tank is getting light; a little algae is not a problem.