Start with a tank size that matches your space and commitment level

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 freshwater aquarium — smaller tanks swing wildly in temperature and chemistry, which stresses fish and kills them faster. A 10-gallon tank can work if you stock it very lightly (three to four small fish), but you will spend more time managing water quality than you would with a larger tank.

Bigger tanks are actually easier to maintain because the water volume buffers against sudden changes. A 40-gallon breeder tank or a standard 55-gallon tank gives you room to add fish later without constant water changes. If you have the floor space and budget, start with 40 gallons or more. If you are renting or have limited space, a 20-gallon long tank (which is wider than it is tall) works well and fits on most stands.

Tank shape matters too. A tall, narrow tank looks dramatic but gives fish less swimming room and less surface area for oxygen exchange. A longer, wider tank is better for most community fish. Avoid decorative bowls and hexagon tanks — they do not hold enough water to stay stable, and they are harder to filter and heat properly.

Key Takeaways

  • A 20-gallon tank is the practical minimum; smaller tanks are harder to keep stable and require more frequent maintenance.
  • You will need a filter rated for your tank size, a heater set to 76–78 degrees Fahrenheit for tropical fish, and an air pump or power head for water movement.
  • Substrate (gravel or sand), plants or decorations, and a water conditioner that removes chlorine are essential before you add any fish.
  • Cycle your tank for two to four weeks before adding fish so beneficial bacteria can establish and break down waste.
  • Start with hardy, peaceful fish like tetras, corydoras catfish, or guppies; avoid aggressive or delicate species until you have experience.

Choose a filter, heater, and air system rated for your tank

A filter is non-negotiable. It removes waste and uneaten food, and it creates surface agitation so oxygen enters the water. Buy a filter rated for at least your tank size — a 20-gallon tank needs a filter rated for 20 gallons or higher. Hang-on-back filters (also called power filters) are cheap and effective for tanks up to 40 gallons. Canister filters cost more but are quieter and more powerful for larger tanks. Sponge filters are cheap and gentle on fish but slow; they work best in small tanks or as a backup.

A heater is essential unless you live in a warm climate and your room stays above 76 degrees year-round. Most tropical fish need 76–78 degrees Fahrenheit. Buy a heater rated for your tank size — a 50-watt heater works for a 10-gallon tank, a 100-watt for a 20-gallon, and a 200-watt for a 40-gallon. Pair it with a thermometer so you can check the temperature daily. Submersible heaters are cheaper than hang-on models and take up less space.

An air pump is optional but useful. It powers an air stone (a porous tube that releases bubbles) and adds oxygen to the water. A filter already creates surface agitation, so an air pump is not strictly necessary, but it helps in tanks with heavy fish load or in summer when water holds less oxygen. A small air pump costs $15 to $30.

Add substrate, decorations, and plants before filling the tank

Substrate is the material on the bottom of the tank. Gravel is the most common choice — it looks natural, does not cloud the water, and lets beneficial bacteria colonize it. Buy aquarium gravel, not playground gravel, because aquarium gravel is rinsed and safe. Plan on 1 to 2 pounds per gallon — a 20-gallon tank needs 20 to 40 pounds. Rinse the gravel in a bucket under running water until the water runs clear, or it will cloud your tank for days.

Sand is another option. It looks more natural and is better for fish that dig (like corydoras catfish), but it compacts over time and can trap waste. If you use sand, buy aquarium sand and rinse it thoroughly — it takes longer to rinse than gravel.

Decorations give fish places to hide and rest. Use rocks, driftwood, or plastic plants. Live plants are better because they consume nitrate (a fish waste product) and produce oxygen, but they need light and sometimes fertilizer. Java fern and anubias are hardy plants that grow slowly and do not need much light. Avoid sharp decorations that can tear fish fins.

Fill the tank and condition the water before adding fish

Fill the tank slowly with tap water. Tap water contains chlorine and chloramine, which kill fish and beneficial bacteria. Buy a water conditioner (also called dechlorinator) and add it according to the bottle instructions — most conditioners work when ready. Some conditioners also bind heavy metals and add a protective slime coat to fish skin.

Turn on the filter and heater. Let the heater run for at least an hour before you add fish so the water reaches the target temperature. Check the thermometer to confirm.

Do not add fish yet. The tank needs to cycle — beneficial bacteria need to colonize the filter media and substrate so they can break down ammonia (from fish waste) into nitrite, then into nitrate. This process takes two to four weeks. You can speed it up by adding a bacterial starter culture (sold as "beneficial bacteria" or "tank starter"), but it still takes at least a week.

Test water chemistry during the cycling period

Buy a water test kit that measures ammonia, nitrite, and nitrate. A liquid test kit is more accurate than test strips, though strips are faster. Test the water every two to three days during cycling.

In the first week, ammonia will rise (usually to 1–2 ppm). In the second week, nitrite will rise and ammonia will drop. By week three or four, ammonia and nitrite should both be near zero, and nitrate should be present (usually 5–20 ppm). When ammonia and nitrite stay at zero for three days in a row, the tank is cycled and ready for fish.

If you do not want to wait, you can do a fish-in cycle — add a few hardy fish and do 25 percent water changes every two to three days to keep ammonia and nitrite low. This stresses the fish, but hardy species like guppies and tetras usually survive it. Most experienced aquarists recommend waiting and doing a fishless cycle instead.

Stock the tank slowly with hardy, peaceful fish

Start with three to five small fish. Good beginner fish include neon tetras, cardinal tetras, guppies, corydoras catfish, and otocinclus catfish. These fish are hardy, peaceful, and do not need special conditions. Avoid goldfish (they need much larger tanks and produce a lot of waste), bettas (males fight each other), and plecos (they grow huge and eat plants).

Add fish gradually — wait a week between additions so the bacteria can adjust to the increased waste load. If you add too many fish at once, ammonia and nitrite will spike and kill them.

Watch the fish for the first few hours after adding them. They should explore the tank and eat food within an hour. If they hide constantly or gasp at the surface, something is wrong — check the temperature, ammonia, and nitrite when ready.

Do weekly water changes and monitor the tank

Once the tank is stocked, do a 25 percent water change every week. Remove water with a siphon or gravel vacuum, then add fresh water that has been treated with conditioner. A gravel vacuum removes waste from the substrate while you drain water, which is more efficient than just siphoning.

Test ammonia, nitrite, and nitrate every two weeks. Ammonia and nitrite should stay at zero. Nitrate will rise over time — when it reaches 40 ppm or higher, do a larger water change (30–50 percent) to bring it back down. High nitrate stresses fish and causes algae blooms.

Feed fish once or twice a day, only as much as they can eat in two to three minutes. Uneaten food rots and raises ammonia. Most beginner fish do fine on basic flake food or pellets.

Frequently Asked Questions

How long does it take to set up a tank before I can add fish?

Plan on two to four weeks if you do a fishless cycle. If you do a fish-in cycle with hardy fish and frequent water changes, you can add fish after the tank has run for three to five days, but the fish will be stressed and some may die. Most people recommend waiting for a full fishless cycle.

Can I use tap water straight from the faucet?

No. Tap water contains chlorine and chloramine, which kill fish and beneficial bacteria. Always treat tap water with a water conditioner before adding it to the tank. Some conditioners work when ready; others need a few minutes. Read the bottle to be sure.

What should I do if my fish are gasping at the surface?

Gasping usually means low oxygen or high ammonia. Check the temperature (it should be 76–78 degrees for tropical fish). Turn on an air pump or increase filter flow. Test ammonia and nitrite when ready — if either is above zero, do a 50 percent water change right away and retest in a few hours.

How often should I clean the filter?

Rinse the filter media in old tank water (not tap water) every two to four weeks, depending on how dirty it looks. Do not use tap water because it will kill the beneficial bacteria. Replace the media only when it falls apart — usually every six months to a year.

Can I keep different types of fish together?

Most small, peaceful fish can live together — tetras, guppies, corydoras catfish, and otocinclus catfish are all compatible. Avoid mixing aggressive fish (like cichlids) with peaceful ones, and do not keep two male bettas in the same tank. Research each species before you buy to make sure they have similar temperature and water chemistry needs.