Start with the fish you want, not the tank size you like

The most common mistake is buying an aquarium first, then trying to fit fish into it. Work backwards instead: decide what fish you want to keep, then buy a tank large enough for them to survive and behave normally. Different fish need different amounts of space, water temperature, and tank conditions—and those needs don't change because you have a smaller tank.

Research the specific fish species you're interested in before you buy anything. Look up their adult size (not the size they are in the pet store), how much space they need per fish, what water temperature they require, and whether they're aggressive toward other fish. A betta fish needs far less space than a goldfish. Tropical fish need a heater; coldwater fish don't. Some fish are solitary; others need to live in groups. These facts determine what tank will actually work.

Key Takeaways

  • Choose your fish species first, then select a tank size that meets their needs—not the other way around.
  • Larger tanks are easier to maintain because water chemistry changes more slowly, making them more forgiving for beginners.
  • A tank's dimensions matter as much as its volume: a tall narrow tank holds the same gallons as a short wide one, but fish need horizontal swimming space.
  • Budget for a filter, heater (if needed), and a water test kit alongside the tank itself, because the tank alone is not enough to keep fish alive.
  • Tap water contains chlorine and chloramine that kill fish; you need a water conditioner before any fish go in the tank.

Tank size: bigger is easier, and 20 gallons is a practical starting point

A larger tank is more stable and more forgiving. In a small tank, uneaten food and fish waste build up quickly and change the water chemistry fast. In a larger tank, the same waste is diluted across more water, so the chemistry stays balanced longer. This gives you more time to notice problems and fix them. For a beginner, a 20-gallon tank is a practical size: it's large enough to be stable, small enough to fit in most homes, and affordable enough that mistakes aren't catastrophically expensive.

That said, tank size depends entirely on what fish you're keeping. A single betta can live in a 5-gallon tank (though 10 gallons is better). A goldfish needs 20 gallons for the first fish and 10 gallons for each additional one—so two goldfish need 30 gallons. Tropical community fish like tetras and guppies can live in a 10-gallon tank if you keep the numbers low. Look up the specific needs of the fish you want before you decide on a size.

Tank shape: length matters more than height

Fish swim horizontally, not vertically. A tall narrow tank and a short wide tank might hold the same number of gallons, but the short wide tank gives fish more usable swimming space. A 20-gallon tall (24 inches tall, 12 inches wide) feels cramped to fish. A 20-gallon long (30 inches long, 12 inches tall) gives them room to move. When you're comparing tanks, check the dimensions, not just the volume.

The footprint—the length and width of the base—is what matters most. Longer tanks let fish swim in straight lines and establish territories. Narrow tanks force them to turn constantly. If you have space for a 40-gallon breeder tank (36 inches long, 18 inches wide) instead of a 40-gallon tall, your fish will be noticeably happier and easier to observe.

Glass versus acrylic: durability, clarity, and cost

Glass tanks are the standard. They're durable, scratch-resistant, and stay clear for years. They're heavier than acrylic, so they need a sturdy stand, but they don't flex or warp. Glass tanks are usually cheaper upfront. The main drawback is that they can crack or shatter if you drop them or if something heavy falls on them.

Acrylic tanks are lighter and won't shatter, which makes them safer in homes with children or pets. They're also easier to move. The trade-off is that acrylic scratches easily—even cleaning it with a soft cloth can leave marks over time. Acrylic also costs more than glass. If you plan to keep the tank in one place and don't expect it to take impacts, glass is the practical choice. If you need something portable or shatterproof, acrylic is worth the extra cost.

Filter and heater: non-negotiable equipment

The tank itself is only part of the system. You also need a filter to remove waste and keep the water clean. Filters come in several types: hang-on-back filters are affordable and straightforward to install; canister filters are powerful and good for larger tanks; sponge filters are quiet and gentle on small fish. The filter should process the entire tank volume at least three times per hour. A 20-gallon tank needs a filter rated for at least 60 gallons per hour.

If you're keeping tropical fish, you need a heater to maintain water temperature between 76 and 80 degrees Fahrenheit. Coldwater fish like goldfish and bettas don't need a heater if your home stays above 70 degrees. A heater with a built-in thermostat turns on and off automatically to keep the temperature stable. Budget for a heater that's sized for your tank—a 50-watt heater works for a 10-gallon tank; a 100-watt heater works for a 20-gallon tank.

You'll also need a water test kit to check ammonia, nitrite, and nitrate levels. These are waste products that build up in the tank and poison fish if they get too high. A basic kit with test strips or liquid tests costs $15 to $30 and lasts for dozens of tests. Don't skip this—you can't see these chemicals, but they kill fish silently.

Water conditioner and cycling: preparing the tank before fish arrive

Tap water contains chlorine and chloramine to kill bacteria for human drinking. These chemicals kill fish. Before you add any fish, treat the water with a water conditioner (also called dechlorinator). Add the conditioner to the water according to the bottle's instructions, wait a few minutes, and the chemicals are neutralized. You'll need to do this every time you add new water to the tank.

Before you add fish, the tank also needs to go through a process called cycling. Beneficial bacteria colonize the filter and substrate and convert fish waste (ammonia) into less toxic compounds (nitrite, then nitrate). This takes two to four weeks. During cycling, test the water every few days. When ammonia and nitrite both read zero and nitrate is present, the tank is cycled and ready for fish. If you add fish before the tank is cycled, ammonia and nitrite will spike and poison them. Some people cycle a tank by adding fish and doing frequent water changes, but that stresses the fish. It's kinder to cycle the tank first, then add fish to a stable environment.

Substrate, plants, and decorations: creating space and hiding places

The bottom of the tank needs substrate—gravel, sand, or soil. Substrate gives beneficial bacteria a place to live and lets you plant live plants if you want them. Gravel is the easiest choice for beginners; it's inexpensive and doesn't compact. Sand looks more natural and is better if you're keeping fish that dig or sift through the bottom, like corydoras catfish. Avoid substrate that's too fine or too coarse; medium-sized gravel works for most setups.

Fish need places to hide and rest. Decorations, plants, and driftwood break up the open space and reduce stress. Live plants also absorb some waste and produce oxygen, which helps keep the water clean. If you don't want to maintain live plants, artificial plants and caves work too. The key is that fish shouldn't be able to see the entire tank from any one spot—that makes them feel exposed and stressed.

Frequently Asked Questions

Can I use a bowl or vase for a fish tank?

No. Bowls and vases have no room for a filter, heater, or hiding places. They also have a very small surface area for oxygen exchange, so the water becomes toxic quickly. Even a betta, which can survive in small spaces, lives longer and behaves better in a 5-gallon tank with a filter than in a bowl. A bowl is not a humane home for any fish.

How much does a basic aquarium setup cost?

A 20-gallon glass tank, filter, heater, substrate, and basic decorations typically cost $150 to $250 total. A water test kit adds $20 to $30. Water conditioner is $10 to $15. The fish themselves cost $5 to $50 each depending on species. Budget for ongoing costs: filter media replacement ($10 to $20 per month), water conditioner for water changes, and electricity for the heater and filter.

Should I buy a kit tank or build my own?

Beginner kits that include a tank, filter, and heater are convenient and usually cost less than buying each piece separately. The trade-off is that kit filters are often smaller and less powerful than standalone filters. If you're willing to research and buy each piece separately, you can get better equipment for the same price. Either approach works; kits are just faster.

What if I want to keep multiple species of fish together?

Some fish live peacefully together; others are aggressive or eat smaller fish. Research whether the species you want are compatible before you buy them. Peaceful community fish like tetras, guppies, and corydoras catfish can live together. Aggressive fish like cichlids and bettas often need to live alone or in specific group setups. Never assume two fish will get along just because they're both fish.

Do I need a stand for the tank, or can I put it on a shelf?

A 20-gallon tank filled with water weighs about 200 pounds. It needs a sturdy, level surface that can support that weight without flexing. A dedicated aquarium stand is designed for this and usually has storage underneath. A shelf might work if it's rated for that weight, but check the shelf's weight limit first. An uneven or weak surface can cause the tank to crack or leak.