Start with a tank size you can actually maintain

The most common mistake beginners make is buying a tank that is too small. A 20-gallon tank is the practical minimum for a freshwater aquarium because it holds water temperature and chemistry more steadily than smaller tanks. Smaller tanks swing between extremes — a few degrees warmer or cooler, a spike in ammonia — and fish die from the instability, not from the size itself.

A 20-gallon tank costs $30 to $80 new, depending on shape. A 10-gallon tank costs less upfront but demands more frequent water changes and more careful stocking. If you are new to this, spend the extra money on the larger tank. You will lose fewer fish and spend less time troubleshooting.

Decide where the tank will live before you buy it. The spot should be away from direct sunlight (which causes algae blooms), away from heating vents and air conditioners, and on a surface that can hold the weight — a 20-gallon tank filled with water and gravel weighs roughly 200 pounds. Once you fill it, moving it is dangerous and damages the seal.

Key Takeaways

  • A 20-gallon tank is easier to maintain than smaller tanks because water temperature and chemistry stay more stable.
  • You need a filter, heater, thermometer, and substrate (gravel or sand) before you add any fish.
  • Tap water contains chlorine that kills beneficial bacteria, so treat it with dechlorinator before it enters the tank.
  • Let the tank run empty for one to two weeks so bacteria can colonize the filter and gravel — this is called cycling, and skipping it kills fish.
  • Start with hardy fish like tetras, danios, or corydoras catfish, not goldfish or bettas, which have different water requirements than most beginner setups.

Gather the equipment before you fill the tank

You need five things before water goes in: a filter, a heater, a thermometer, substrate (gravel or sand), and a light. A basic setup costs $100 to $200 total.

The filter removes waste and dead plant matter. A hang-on-back filter (the most common type) clips to the rim and costs $20 to $40. It should process the entire tank volume at least three times per hour — for a 20-gallon tank, look for a filter rated 60 gallons per hour or higher. The filter comes with a cartridge that you replace every month or two.

The heater keeps water at a steady temperature. Most tropical fish need 76 to 78 degrees Fahrenheit. A submersible heater (one you place inside the tank) costs $15 to $30 and comes in wattages from 50 to 300 watts. For a 20-gallon tank, a 100-watt heater is standard. Plug it into a timer so it does not run constantly.

A thermometer sticks to the inside glass and costs $3 to $8. Check it daily for the first month so you know whether the heater is holding temperature.

Substrate is the material on the bottom. Aquarium gravel costs $8 to $15 per bag and looks better than sand, but sand ($5 to $10 per bag) is easier to clean and better for bottom-feeding fish. You need roughly 1 pound per gallon, so 20 pounds for a 20-gallon tank. Rinse it in a bucket under running water three times before it goes in — it is dusty out of the bag.

A light costs $20 to $50 and runs 8 to 10 hours per day. If you do not plan to keep live plants, a basic LED strip is enough. If you do, buy a light rated for plants.

Treat the water and let the tank cycle

Tap water contains chlorine and chloramine, which kill fish and the bacteria you need in the filter. Before you add water, buy a dechlorinator — a liquid or powder that removes these chemicals. It costs $5 to $10 and lasts for months. Follow the bottle's instructions for how much to add per gallon.

Fill the tank with treated water, turn on the filter and heater, and do nothing else for one to two weeks. This waiting period is called cycling, and it is the most important step you will take. During this time, beneficial bacteria colonize the filter cartridge and gravel. These bacteria convert ammonia (fish waste) into nitrite, then into nitrate, which is less toxic. Without this bacteria, ammonia spikes and kills fish within days.

You cannot see cycling happen. You can test for it by buying a water test kit ($20 to $30) and checking ammonia, nitrite, and nitrate levels every few days. When ammonia and nitrite both read zero and nitrate reads above 5, the tank is cycled. If you do not want to test, straightforward wait two weeks — most tanks cycle in that time.

Do not add fish during cycling. Do not add decorations, plants, or anything else. Just let the water run.

Stock the tank slowly with hardy fish

After cycling is complete, add fish gradually — no more than a handful at a time, spaced a week apart. This gives the bacteria time to adjust to the extra waste.

Start with hardy fish that tolerate beginner mistakes. Neon tetras (small, colorful, $1 to $2 each) do well in groups of six or more. Zebra danios (striped, active, $2 to $3 each) also school and are forgiving. Corydoras catfish (bottom-feeders, peaceful, $3 to $5 each) eat leftover food and keep the gravel clean. These fish live in water between 72 and 78 degrees and do not need special care.

Avoid goldfish, bettas, and plecos in a 20-gallon tank. Goldfish need 20 gallons per fish and prefer cold water (65 to 68 degrees). Bettas are aggressive and need their own tank. Plecos grow to 12 inches and outgrow most beginner setups.

A good rule: one inch of fish per gallon of water. A 20-gallon tank can hold 20 inches of fish — that might be ten 2-inch tetras, or six 3-inch danios, or a mix. Do not exceed this, or waste builds up faster than the filter can handle.

Perform weekly water changes and monitor the tank

Every week, remove 25 percent of the water and replace it with fresh, treated water. This removes nitrate (which builds up over time) and keeps the tank stable. Use a siphon or a bucket — do not remove the fish. This takes 15 minutes and is the main work of keeping an aquarium.

Check the thermometer every day for the first month. After that, check it once a week. If the temperature drops below 74 or rises above 80, adjust the heater or move the tank away from the heat source.

Replace the filter cartridge every four to six weeks, or when water flow slows. A clogged cartridge reduces oxygen in the tank and stresses fish.

Watch the fish for signs of stress: gasping at the surface, hiding constantly, or refusing food. These usually mean ammonia or nitrite is too high, which happens if you overfeed or skip water changes. Do a 50 percent water change when ready and reduce feeding.

Feed sparingly and keep the tank clean

Feed fish once or twice a day, only as much as they can eat in two minutes. Uneaten food rots and spikes ammonia. Most beginners overfeed — it is the leading cause of dead fish. A small pinch of flakes is enough for a handful of tetras.

Use a net to remove dead fish or plants when ready. Decaying matter fouls the water. Check the gravel for trapped food or waste and use a gravel vacuum (a siphon with a wide tube) during water changes to suck it out.

If algae grows on the glass, scrape it with a plastic scraper (not metal, which scratches). A small amount of algae is normal and harmless. If it covers the glass in days, you are overfeeding or the light is on too long — reduce feeding or cut light to 8 hours per day.

Troubleshoot common problems early

Cloudy water in the first week usually means the tank is still cycling — wait a few more days. Cloudy water after the tank is established means bacteria are blooming, which happens if you overfeed or skip water changes. Do a 50 percent water change and reduce feeding.

Fish gasping at the surface means low oxygen. This happens if the filter is clogged, the heater is too high, or the tank is overstocked. Replace the filter cartridge, lower the temperature to 76 degrees, and do a water change.

Fish hiding or refusing food for more than a day usually means ammonia or nitrite is high. Test the water if you have a kit, or do a 50 percent water change as a precaution. If the problem persists, the tank may not be fully cycled — wait another week before adding more fish.

White spots on fish (ich) are a parasite that spreads fast. Raise the temperature to 80 degrees, do daily water changes, and buy ich treatment from a pet store. Follow the bottle's instructions exactly.

Frequently Asked Questions

Can I use tap water straight from the faucet?

No. Tap water contains chlorine and chloramine that kill fish and beneficial bacteria. Always treat it with dechlorinator before it enters the tank. The treatment is inexpensive and takes seconds — just add the dose to the water and wait a few minutes before pouring it in.

How long does it take before I can add fish?

One to two weeks. This is the cycling period, when bacteria colonize the filter and gravel. If you add fish before cycling is complete, ammonia spikes and kills them. There is no way to speed this up safely — patience is the only option.

What if I forget to do a water change?

One missed week usually does not harm the tank. If you miss two or more weeks, nitrate builds up and fish become stressed. Do a 50 percent water change as soon as you remember, then return to the weekly schedule. Set a phone reminder if you tend to forget.

Can I keep a betta in a 20-gallon tank with other fish?

Most bettas attack other fish, especially males. Some females tolerate tank mates in a heavily planted 20-gallon setup, but it is risky. If you want a betta, give it its own 5-gallon tank. If you want community fish, choose tetras, danios, or corydoras instead.

What should I do if a fish dies?

Remove it when ready with a net. A dead fish decays and spills ammonia into the water. Do not replace it right away — wait a week so the bacteria adjust to the lower bioload. If multiple fish die in a short time, test the water or do a large water change and check that the filter is running.