Start with the tank size and location that match your space and fish

The first decision is how large a tank you can realistically 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. Smaller tanks (10 gallons or less) swing wildly in temperature and chemistry, making them harder to keep balanced. Larger tanks are more forgiving but take up more space and cost more to fill and heat.

Choose a location away from direct sunlight, heating vents, and air conditioning units. Sunlight causes algae blooms. Temperature swings from vents and AC stress fish. The spot should be on a sturdy, level surface—a 20-gallon tank filled with water weighs roughly 170 pounds. Check that your floor can handle it, especially in older homes or upper floors. Keep the tank away from high-traffic areas where it might get bumped.

Key Takeaways

  • A 20-gallon tank is easier to maintain than smaller tanks because water chemistry stays more stable, and it costs less than very large setups.
  • You need a filter, heater, thermometer, and substrate (gravel or sand) before you add any fish—these are not optional extras.
  • Let the tank run empty for 24 hours after setup, then run it for 3 to 4 weeks before adding fish so beneficial bacteria can establish.
  • Start with hardy fish like tetras or danios that tolerate beginner mistakes, and add only a few fish at a time so the filter can handle the waste.
  • Test water weekly for ammonia and nitrite during the first month, and every two weeks after that to catch problems early.

Gather the essential equipment before filling the tank

You will need a filter, a heater, a thermometer, substrate (gravel or sand), and a light. Do not skip the filter or heater—they are not luxuries. The filter removes waste and grows the bacteria that keep ammonia from poisoning your fish. The heater holds temperature steady; most tropical fish need 76 to 78 degrees Fahrenheit. A thermometer lets you verify the heater is working.

Substrate is the material on the bottom of the tank. Standard aquarium gravel works well and is inexpensive. Sand looks more natural but traps waste and needs more maintenance. Rinse gravel thoroughly under running water before adding it—it comes dirty from the factory. A light helps you see the fish and supports plant growth if you add plants later. A basic hood with a fluorescent bulb is sufficient.

You will also need a water conditioner (removes chlorine from tap water), a bucket dedicated only to the aquarium (never use one that held chemicals), and a siphon or gravel vacuum for water changes. A net for catching fish is useful but not essential at the start. Buy a test kit that measures ammonia, nitrite, and nitrate—the liquid test kits are more accurate than strips, though strips are faster.

Assemble the tank and run it empty for 24 hours

Rinse the tank inside and out with water only—no soap. Place it on the level surface where it will stay. Spread the rinsed substrate evenly on the bottom, about 2 inches deep. Position the heater and filter intake according to their instructions. Most filters hang on the back or sit inside the tank. The heater usually goes in a corner where water flow will carry warm water around the tank.

Fill the tank slowly with tap water treated with water conditioner. Pour water onto a plate or into your hand to avoid stirring up the substrate. Once full, plug in the filter and heater but do not add fish yet. Let everything run for 24 hours. Check that water is flowing, the heater is warming the water, and there are no leaks. The thermometer should read close to 76 to 78 degrees within a few hours.

Cycle the tank for 3 to 4 weeks before adding fish

Cycling means letting beneficial bacteria establish in the filter so they can break down fish waste. Without this step, ammonia builds up and poisons fish. You have two options: fishless cycling or fish-in cycling. Fishless cycling is safer for the fish but takes longer. Fish-in cycling is faster but stresses the fish.

For fishless cycling, add ammonia (pure ammonia from a hardware store, not ammonia cleaner) to bring the level to 2 parts per million. Test daily with your kit. Bacteria will consume the ammonia and produce nitrite. When nitrite appears, bacteria that eat nitrite will start growing. When both ammonia and nitrite drop to zero and nitrate appears, the cycle is complete—usually 3 to 4 weeks. Then do a 50 percent water change to lower nitrate, and you are ready for fish.

For fish-in cycling, add a small number of hardy fish (3 to 5 small fish in a 20-gallon tank) and test ammonia and nitrite every other day. Do a 25 percent water change whenever ammonia or nitrite rises above 0.5 parts per million. The cycle still takes 3 to 4 weeks, but water changes protect the fish from poisoning. This method is riskier because fish are exposed to ammonia and nitrite, but it works if you stay on top of testing and changes.

Choose hardy fish and add them slowly

Start with fish that tolerate beginner mistakes: neon tetras, cardinal tetras, zebra danios, or corydoras catfish. Avoid fancy goldfish (they need much larger tanks and produce enormous amounts of waste), bettas (they fight other fish), and delicate fish like discus or seahorses. Check the maximum adult size of any fish you choose—a fish that grows to 4 inches needs more space than one that stays at 1 inch.

Add only 3 to 5 fish at a time, even if your tank could theoretically hold more. The filter bacteria need time to grow as the fish waste increases. Wait a week between additions. This slow approach prevents ammonia spikes that kill fish. A common rule is 1 inch of fish per gallon of water, but this is a rough guide—it depends on the fish species and how much you feed them.

When you bring fish home, float the bag in the tank for 15 minutes so the water temperature inside the bag matches the tank. Then slowly add tank water to the bag over 10 minutes, letting the fish adjust to the new chemistry. Finally, gently pour the fish into the tank. Do not pour the bag water into the tank—it may carry disease or chemicals from the store.

Feed sparingly and test water regularly

Feed only what the fish will eat in 2 to 3 minutes, once or twice a day. Uneaten food rots and fouls the water. Most beginners overfeed. If you miss a day of feeding, the fish will be fine—they can go a week without food. If you overfeed, ammonia spikes and fish die. Start with a small pinch and watch how much they eat.

Test ammonia and nitrite weekly for the first month after adding fish. Once both stay at zero for two weeks, you can test every two weeks. Keep a log of the results so you can spot trends. If ammonia or nitrite rises, do a 25 percent water change when ready and retest in a few days. Nitrate will slowly climb over time—when it reaches 40 parts per million, do a 25 percent water change to bring it down.

Perform regular maintenance to keep the tank stable

Change 25 percent of the water every two weeks. Use the siphon to remove water from the bottom of the tank, where waste settles. This removes nitrate and keeps the water clean without disrupting the filter bacteria. Refill with fresh water treated with water conditioner at the same temperature as the tank.

Clean the filter according to its instructions—usually every two to four weeks. Rinse the filter media (sponge or cartridge) in old tank water, not tap water, because chlorine kills the bacteria you need. Never replace the entire filter at once; if you must change the media, do it gradually so bacteria can recolonize the new material.

Check the heater and thermometer weekly to make sure temperature stays steady. Wipe algae off the glass with a soft cloth or algae scraper. If algae becomes heavy, reduce feeding and light hours—algae thrives on excess nutrients and light. A few algae-eating fish like plecos or otocinclus catfish can help, but they are not a substitute for good maintenance.

Frequently Asked Questions

How long does it take to set up an aquarium from start to adding fish?

About 4 to 5 weeks. The first 24 hours is assembly and testing. The next 3 to 4 weeks is cycling the tank so bacteria establish. Once ammonia and nitrite are zero, you can add fish. If you are impatient, fish-in cycling is faster, but it stresses the fish.

Can I use tap water straight from the faucet?

No. Tap water contains chlorine and chloramine, which kill fish and beneficial bacteria. Always treat it with a water conditioner before adding it to the tank. Conditioner removes chlorine when ready, so you can add treated water right away.

What should I do if fish start dying?

Test ammonia, nitrite, and nitrate when ready. If ammonia or nitrite is above zero, do a 25 percent water change and retest in a few hours. If both are zero, check temperature (should be 76 to 78 degrees) and make sure you are not overfeeding. If fish are gasping at the surface, the filter may be clogged or the heater may have failed.

Do I need live plants in the aquarium?

No, but they help. Live plants consume nitrate and produce oxygen, making the tank easier to maintain. They also give fish places to hide and look natural. Start with hardy plants like java fern or anubias if you want to try them.

How often should I completely change the water?

Never. A complete water change removes the beneficial bacteria and destabilizes the tank. Stick to 25 percent changes every two weeks. The only reason to do a complete change is if there is a disease outbreak, and even then, you should move the fish to a separate tank first.