What You Need Before You Start

A working freshwater aquarium needs six things: a tank, a filter, a heater, substrate (gravel or sand), water conditioner, and a light. You do not need an air pump if your filter creates surface movement. You do not need plants, decorations, or expensive equipment to keep fish alive. Start with these basics, and add extras later once the tank is running.

Tank size matters more than most beginners think. A 20-gallon tank is the smallest size that forgives beginner mistakes — it holds temperature and water chemistry more steadily than a 10-gallon. A 10-gallon works, but it requires more frequent water changes and closer attention. Avoid anything smaller than 10 gallons for your first tank.

The filter must move the tank's full volume at least three times per hour. A 20-gallon tank needs a filter rated for 60 gallons per hour or higher. Check the filter's packaging for its flow rate in gallons per hour (GPH). The heater should be sized to your tank — roughly 3 to 5 watts per gallon. A 20-gallon tank needs a 75 to 100-watt heater.

Key Takeaways

  • A 20-gallon tank with a filter, heater, substrate, and water conditioner is the minimum setup that works reliably for beginners.
  • The filter must process the tank's full volume at least three times per hour, and the heater should provide 3 to 5 watts per gallon of water.
  • Tap water contains chlorine and chloramine that kill beneficial bacteria, so treat it with water conditioner before adding it to the tank.
  • The nitrogen cycle takes two to four weeks to establish, during which ammonia and nitrite levels spike — test water regularly and do partial water changes if either reading climbs above 0.5 ppm.
  • Start with hardy fish like tetras, guppies, or corydoras catfish, and add only a few at a time so the filter bacteria can catch up.

Assembling the Tank and Running the Filter

Rinse the tank with water only — no soap. Place it on a sturdy, level surface that can hold the weight. A 20-gallon tank filled with water weighs about 170 pounds. Rinse the substrate under running water until the water runs clear, then spread it 2 to 3 inches deep across the tank bottom.

Fill the tank halfway with tap water. Place the filter intake in the water and plug it in. The filter will run continuously from this point forward. Add the heater and set it to 76 to 78 degrees Fahrenheit — a safe range for most beginner fish. Let the heater warm the water for an hour before adding anything else.

While the heater works, treat the remaining tap water with water conditioner. Chlorine and chloramine in tap water kill the beneficial bacteria your tank needs. Follow the conditioner's dosing instructions on the bottle. Fill the tank the rest of the way with treated water, then let the filter run for at least 24 hours before adding fish.

Understanding the Nitrogen Cycle

Fish waste breaks down into ammonia, which is toxic. Beneficial bacteria convert ammonia to nitrite (also toxic), then to nitrate (less toxic). This process is called the nitrogen cycle, and it takes two to four weeks to establish. During this time, ammonia and nitrite levels will spike, then fall as bacteria populations grow.

Test the water every two to three days using a liquid test kit — not test strips, which are less accurate. You need to measure ammonia, nitrite, and nitrate. Ammonia and nitrite should both read 0 ppm once the cycle is complete. Nitrate will climb to somewhere between 5 and 20 ppm, which is normal.

If ammonia or nitrite climbs above 0.5 ppm, do a 25 percent water change. Remove one-quarter of the water and replace it with treated tap water at the same temperature. This dilutes the toxins without disrupting the bacteria. Do not clean the filter during this period — the bacteria live in the filter media.

Choosing and Adding Your First Fish

Wait until ammonia and nitrite both read 0 ppm before adding any fish. This usually takes two to four weeks. Once the cycle is complete, start with hardy species that tolerate beginner mistakes: neon tetras, guppies, corydoras catfish, or platies. Avoid goldfish, which produce far more waste than beginners expect, and avoid cichlids, which are aggressive and need larger tanks.

Add only a few fish at a time — no more than three to five in the first week. The bacteria population grows to match the fish load, so adding too many at once will cause ammonia and nitrite to spike again. Wait a week between additions, then test the water before adding more.

Acclimate new fish before releasing them into the tank. Float the bag they came in on the water surface for 15 minutes so the temperature inside the bag matches the tank temperature. Then open the bag and slowly add tank water to it over the next 15 minutes, one cup at a time. This prevents shock from sudden changes in water chemistry. Use a net to transfer the fish into the tank, and discard the bag water.

Weekly and Monthly Maintenance

Test the water once a week after the cycle is established. Ammonia and nitrite should stay at 0 ppm. Nitrate will climb over time and should stay below 40 ppm. If nitrate climbs above 40 ppm, do a 25 percent water change.

Do a 25 percent water change every two weeks. Siphon out one-quarter of the water using a gravel vacuum, which removes debris from the substrate at the same time. Replace it with treated tap water at the same temperature. This removes excess nitrate and replenishes minerals the fish consume.

Clean the filter once a month by rinsing the filter media in a bucket of old tank water — never tap water, which kills the bacteria. Squeeze the media gently until the water runs clear. The media should never be fully clean; some brown material is the bacteria you need. Replace the filter cartridge only when water flow slows noticeably, usually every three to six months depending on the filter type.

Common Beginner Mistakes to Avoid

Overfeeding is the most common mistake. Feed only what the fish can eat in two to three minutes, once or twice a day. Uneaten food decays and raises ammonia. Hungry fish are healthier than overfed ones.

Changing the water too often or too much will crash the cycle. The bacteria live in the filter and substrate, not in the water itself, but frequent large water changes remove nitrate faster than it builds up, which can starve the bacteria. Stick to 25 percent changes every two weeks.

Cleaning the filter in tap water kills the bacteria. Always rinse filter media in old tank water or dechlorinated water. If you accidentally kill the bacteria, the cycle restarts and ammonia and nitrite will spike again.

Adding fish before the cycle is complete causes ammonia and nitrite spikes that kill the fish. Wait for both readings to hit 0 ppm, even if it takes four weeks. A cycled tank is the foundation of everything that follows.

Equipment Brands and Where to Buy

Aquarium filters come from Aqueon, Marineland, Fluval, and Tetra. All are reliable at their price point. A basic hang-on-back filter for a 20-gallon tank costs between $25 and $50. Heaters from Aqueon and Tetra are standard and cost $15 to $30.

Water test kits from API (the Freshwater Master Kit) are the standard for beginners and cost about $30. It includes ammonia, nitrite, and nitrate tests and lasts for hundreds of tests. Liquid test kits are more accurate than strips and worth the extra cost.

Water conditioner brands like Seachem Prime and Aqueon work identically — they neutralize chlorine and chloramine. A bottle costs $8 to $12 and lasts several months. Buy it wherever you buy the tank: pet stores, online retailers, or big-box stores all stock the same brands.

Frequently Asked Questions

Can I use bottled water instead of tap water?

Bottled water lacks minerals that fish need and is expensive for large tanks. Tap water treated with conditioner is cheaper and better. If your tap water is very hard or very soft, you can mix bottled and tap water, but pure bottled water alone will not support fish long-term.

How long until I can add fish?

Wait until ammonia and nitrite both read 0 ppm on a liquid test kit. This usually takes two to four weeks. Testing every two to three days tells you when the cycle is complete. Do not guess — adding fish too early kills them and restarts the cycle.

What temperature should the water be?

Most beginner fish thrive between 76 and 78 degrees Fahrenheit. Set the heater to 77 degrees and let it stabilize. Do not change the temperature frequently — fish stress when temperature swings more than a few degrees in a day.

Can I use gravel from outside?

No. Outside gravel may contain parasites, chemicals, or bacteria that harm fish. Use aquarium substrate from a pet store. It is inexpensive and safe. Rinse it thoroughly before adding it to the tank.

What if the filter is too loud?

A new filter is often loud because air is trapped in the media. Run it for 24 hours and the noise usually stops. If it stays loud, the intake may be clogged or the filter may be too large for the tank. Check the intake tube and reduce the flow rate if the filter has a valve.