Start with a tank size that matches your space and fish choice

The first decision is tank size, and it matters more than most beginners think. A 20-gallon tank is the practical minimum for a stable aquarium—smaller tanks swing wildly in temperature and chemistry, making fish harder to keep alive. A 10-gallon tank works if you commit to frequent water changes and choose only hardy, small fish like bettas or guppies. Larger tanks (30 gallons and up) are actually easier to maintain because the water chemistry stays steadier.

Where you place the tank affects what you'll need to do later. Pick a spot away from direct sunlight, which causes algae blooms, and away from heating vents or air conditioners that cause temperature swings. Make sure the surface is level and can hold the weight—a 20-gallon tank filled with water, gravel, and decorations weighs roughly 200 pounds. Check that you have an electrical outlet nearby for the filter and heater.

Key Takeaways

  • A 20-gallon tank is the practical starting size; smaller tanks require more frequent maintenance and are harder to keep stable.
  • You need a filter, heater, and thermometer before you add any fish—these run the tank's life support, not just make it look nice.
  • Tap water contains chlorine that kills fish; you must treat it with a dechlorinator or let it sit uncovered for 24 hours before adding fish.
  • The tank must cycle for one to two weeks before fish go in, meaning beneficial bacteria need time to colonize the filter and gravel.
  • Start with hardy, slow-growing fish like tetras or corydoras catfish rather than fancy varieties that need perfect conditions.

Gather the essential equipment before filling the tank

A working aquarium needs three core pieces of equipment: a filter, a heater, and a thermometer. The filter removes waste and dead matter; without it, ammonia builds up and poisons the fish. A heater keeps tropical fish at 75–80 degrees Fahrenheit (most freshwater fish need warmth). The thermometer lets you check that the heater is actually working.

Buy a filter rated for at least the size of your tank—a 20-gallon tank needs a filter rated for 20 gallons or larger. Hang-on-back filters and canister filters both work well for beginners. You'll also need an air pump or airstone if you want surface movement (this helps oxygen dissolve into the water), though a good filter creates enough movement on its own.

Get a dechlorinator bottle before you fill the tank. Tap water contains chlorine to kill bacteria for human use, but it kills fish and the beneficial bacteria you need in the filter. You add a few drops per gallon, or you can let tap water sit uncovered for 24 hours and the chlorine evaporates on its own. A test kit for ammonia, nitrite, and nitrate costs $20–$30 and tells you when the tank is ready for fish and when water changes are needed.

Layer the substrate and add water carefully

Substrate is the material on the bottom of the tank—usually gravel or sand. Rinse it thoroughly under running water until the water runs clear; unrinsed substrate clouds the tank for days. Spread 2 to 3 inches of rinsed substrate across the bottom. This gives beneficial bacteria a place to live and lets plants (if you add them) take root.

Fill the tank slowly. Place a plate or bowl on the substrate and pour water onto it so the water spreads gently instead of blasting the gravel into clouds. Fill the tank three-quarters full, then add your filter, heater, and thermometer. Plug in the filter and heater and let them run for a few minutes to make sure they work. Add dechlorinator to the water according to the bottle's instructions.

Top off the tank to the fill line (usually an inch or two below the rim). Turn on all equipment and let it run. The water will be cloudy for a day or two as particles settle—this is normal and clears on its own.

Let the tank cycle before adding fish

This step separates aquariums that stay healthy from ones where fish die for no obvious reason. Cycling means letting beneficial bacteria colonize the filter and gravel. These bacteria convert ammonia (fish waste) into nitrite, then into nitrate, which is much less toxic. Without this bacterial layer, ammonia builds up and kills fish within days.

A tank cycles in one to two weeks. During this time, run the filter and heater continuously. You can add a small pinch of fish food once a day—it decays and creates ammonia for the bacteria to feed on. Test the water every few days with your ammonia and nitrite kit. When ammonia and nitrite both read zero and nitrate reads between 5 and 20 parts per million, the tank is cycled and ready for fish.

Some people add a commercial bacteria starter to speed this up, though it is not required. If you do use one, follow the bottle's instructions exactly.

Choose hardy fish that match your tank size

Start with fish that forgive beginner mistakes. Neon tetras, guppies, corydoras catfish, and danios are all hardy, colorful, and stay small. Avoid fancy goldfish (they need 40+ gallons and produce enormous waste), plecos (they grow to 12 inches and need driftwood), and cichlids (they are aggressive and need specific tank mates).

Check how many fish your tank can hold. A rough rule is one inch of fish per gallon—a 20-gallon tank holds about 20 inches of fish total. A 2-inch neon tetra counts as 2 inches; a 3-inch corydoras counts as 3 inches. This is a starting point, not a hard limit, but overstocking causes ammonia spikes and dead fish.

Buy fish from a store where the tanks look clean and the fish look alert. Avoid fish with torn fins, spots, or cloudy eyes—these are sick and will infect your tank. Bring them home in a bag and float the bag in your tank for 15 minutes so the water temperature inside the bag matches the tank temperature. Then gently pour the fish into the tank.

Feed sparingly and watch for problems in the first week

Feed fish once a day, only as much as they eat in two minutes. Uneaten food decays and fouls the water. Most beginners overfeed because fish always look hungry—they are not; they are just begging. A small pinch per fish is enough.

In the first week, watch for signs of stress: fish gasping at the surface (not enough oxygen), hiding constantly (water chemistry is off), or torn fins (aggression or disease). If you see these, do a 25 percent water change when ready—remove a quarter of the water and replace it with fresh, dechlorinated water. This dilutes any ammonia or nitrite that may have spiked.

Test the water again after three days. Ammonia and nitrite should still be zero. If they spike above 0.5 parts per million, do another 25 percent water change and check that the filter is running and not clogged.

Maintain the tank with weekly water changes and filter cleaning

Once the tank is stable, do a 25 percent water change every week. Siphon out a quarter of the water (a gravel vacuum makes this straightforward) and replace it with fresh, dechlorinated water at the same temperature. This removes nitrate that builds up over time and keeps the water clean.

Clean the filter every two to four weeks. If you have a hang-on-back filter, rinse the cartridge in old tank water (not tap water, which kills the bacteria). If you have a canister filter, follow the manual's instructions. Never use hot water or soap—both kill the beneficial bacteria you need.

Test the water once a month after the first month. Ammonia and nitrite should always be zero. Nitrate should stay below 40 parts per million; if it climbs higher, increase water changes to 30 or 40 percent per week.

Frequently Asked Questions

Can I use tap water straight from the faucet?

Not without treating it first. Tap water contains chlorine that kills fish and beneficial bacteria. Use a dechlorinator bottle (add a few drops per gallon) or let the water sit uncovered for 24 hours so the chlorine evaporates. After that, the water is safe.

How long does it take before I can add fish?

One to two weeks. The tank must cycle first, meaning beneficial bacteria need time to colonize the filter and gravel. Test the water with an ammonia and nitrite kit; when both read zero, the tank is ready. Adding fish before cycling causes ammonia poisoning and dead fish within days.

What if my fish die shortly after I add them?

The most common cause is ammonia or nitrite spike, which happens if the tank was not cycled long enough or if you added too many fish at once. Do a 25 percent water change when ready and test the water. Add only a few fish at a time and wait a week between additions so the bacteria can adjust.

Do I need live plants in my aquarium?

No, but they help. Live plants consume nitrate and produce oxygen, which makes the tank easier to maintain. straightforward plants like java fern and anubias need only low light and no special fertilizer. Plastic plants work fine if you prefer them, but they do not help with water chemistry.

How often should I clean the entire tank?

Never do a complete drain and scrub unless something goes wrong (like a disease outbreak). Weekly 25 percent water changes and monthly filter cleaning are all a healthy tank needs. Draining the tank kills the beneficial bacteria and forces you to cycle all over again.