How Many Fish in A 20 Gallon Tank? (With 5 Stocking Ideas)

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by Jason Matthews



A 20-gallon tank is a great size for a beginner fish keeper, as it is not too large or small and does not require much maintenance. It can accommodate a wide variety of fish, from small to medium-sized. But do you know how many fish you can put in your 20-gallon tank?

A good rule of thumb is to allow 1 gallon of water per inch of fish. So, a 20-gallon tank could accommodate approximately twenty 1-inch fish. But, it also depends on the breed and adult size of the fish. Only one large goldfish can be kept in a 20-gallon tank, whereas 20 Neon Tetras can be housed as this is a small fish breed.

Generally, starting with a smaller number of fish is best, and adding more over time as your tank becomes established. This will allow your tank to develop the necessary bacteria to break down fish waste and keep the water quality high. A 20-gallon tank can typically accommodate 5-10 small fish or 3-5 medium-sized fish.

This article will discuss some of the best fish for a 20-gallon tank, as well as how many of each you can keep. Keep reading to find out.

Key Takeaways

  • In a 20-gallon tank, you can have a lot of fish. The number of fish you can have in a 20-gallon tank depends on the type of fish and their adult size. A good rule of thumb is to allow 1 gallon of water per inch of fish. 
  • It is important to consider their compatibility when stocking multiple species of fish. Some fish may get along well, while others may be aggressive and disturb other fish in the tank. 
  • Some good choices to put in your 20-gallon tank are Guppies, Neon Tetras, Danios, and Scarlet Badis.

How Many Fish Can You Keep In A 20-Gallon Tank?

FishCare levelSizeDietTemperamentpHTempNumbers
Platies Easy2-3 inchesOmnivorousPeaceful6.8-8.070-82°F12
Tetras Easy1.5 inchesOmnivorousPeaceful6.8-7.875-80°FUp to 20
Guppies Easy1.4-2.4 inchesOmnivorousPeaceful6.8-7.872-78°F12
Rasboras EasyUp to 2 inchesOmnivorousPeaceful6.8-7.875-80°F20
Barbs Easy2.8-3.9 inchesOmnivorousSemi-aggressive6.0-8.077-82°F6
BettasEasy2.25 inchesCarnivorous Aggressive6.5-7.575-80°F1 male /1-5 females
Common MollyEasyUp to 4.5 inchesOmnivorousPeaceful7.5-8.572-78°F4-6
Danios EasyUp to 2 inchesOmnivorousPeaceful7.0-7.870-78°F12
how many fish stock in 20 gallon tank
Image Credit: codywu72, Pixabay

Here we’ll discuss how many fish can be kept in a 20-gallon tank. The size and kind of fish determine how many you may have. Some smaller fish can be kept in this size tank, but other larger fish will need a much bigger tank.


The colorful guppies are a real visual feast in a heavily planted 20-gallon fish tank. By stocking your nano aquarium with some fancy guppies, you’ll have difficulty taking your eyes off them.

If you’re looking to add guppies to your tank, a 20-gallon aquarium can comfortably house 12 fish. However, you should keep between 10 and 12 guppies in the tank to give them plenty of room to swim. If you’re planning on housing your guppies with other fish, 6 to 8 is the ideal number.


Mollies are a type of livebearer and can be kept in a 20-gallon tank. However, other livebearers like platys and guppies do better than mollies in a 20-gallon tank.

If you must want mollies, adding three female mollies and one male molly fish to the tank is the best option. If you want to add a little more action to your fish tank, try pairing them with a school of neon tetras.

Mollies prefer hard water with a little salinity, so they may not be the best choice for those who do not have a saltwater tank. If you keep mollies with neon tetras, ensure the water conditions are compatible.

Neon Tetras

You can keep 20 Neon Tetras in a 20-gallon aquarium, but if you are keeping them with other tropical fish, it’s recommended to go with 10 individuals. They’re quite sensitive to water quality and should only be added to a fully cycled aquarium.

They prefer moderately acidic water that is darkened with tannins akin (similar to their wild habitat).


Dwarf cichlids are a great choice for a 20-gallon tank. They come in a variety of colors and patterns. These fish typically range in size from 3 to 3.5 inches, so they’ll be a perfect fit for a smaller tank. Some of the most popular choices for dwarf cichlid include Golden Dwarf Cichlid, Cockatoo Cichlid, German Blue Ram, and Kribensis Cichlid.

Rainbow Kribs:  A tank of at least 20 gallons is recommended for a pair of these cichlids.

Apistogramma Cichlids: A pair, or a trio of these cichlids, may be housed in a 20-gallon tank, usually residing in pairs or groups with one male and two (or more) females.

Julidochromis: 3 to 6 Julidochromis may be kept in a 20-gallon aquarium as long as there is enough plant cover, rock work, and caves.


Many people believe that you can only keep one betta in a tank. However, if you have a 20-gallon tank, you can safely house in the sorority of up to 5 female bettas. This is because they are less aggressive in a group, and they will have more space to swim.

Just be sure to add plenty of plants and decorations to help break up the space and give them their territory.

Related: See our recommended filters for 20 gallon tanks

What Is the Biggest Fish You Can Put In A 20-Gallon Tank?

A 20-gallon tank may seem like a lot of space, but it’s important to remember that different fish have different needs. For example, Roseline Sharks are very active and need a lot of room to swim and turn around.

Choose your fish carefully to have the appropriate amount of space for their needs.

A 20-gallon tank is best for fish that grow no bigger than six inches, using the one-inch per gallon rule for multiple specimens.

The Best Fish For A 20-Gallon Tank

species for 20 gallon tank
Image Credit: Irina_kukuts, pixabay

Guppies, Betta fish, Neon Tetra, Mollies, Platy fish, and Rasboras are some of the fishes that will make them a fantastic choice for your 20-gallon tank. For simplicity’s sake, we’ve organized the list of fish into categories to help you decide which ones to add to your aquarium.

Community fish

Schooling fish

  • Small Tetras
  • Rasboras
  • Corydoras Catfish

Aggressive fish

  • Bettas (Siamese fighting fish)

Big fish

  • Dwarf Cichlids
  • Gourami

Stocking Ideas for Your 20-Gallon Fish Tank

Many possible combinations of fish can be kept in a 20-gallon tank, but we’ll discuss some of the more popular ones here. These are just suggestions, so you can use them as a starting point to create your own ideal tank.

Livebearer Community Tank

  • 3 Guppies
  • 2 Platies
  • 2 Mollies

Tetra Community Tank

  • 5 Neon tetras
  • 5 Glowlight tetras
  • 3 Black Phantom tetras
  • 3 Cherry shrimp

The Betta Tank

  • 1 Betta Fish
  • 10-15 Harlequin rasbora
  • 4 Panda Corydoras Catfish

The Ram Tank

  • One pair (m/f) Electric Blue Rams
  • 10-15 Ember Tetra
  • 4-6 Albino Corydoras Catfish

The Gourami Tank

  • One pair (m/f) Dwarf Gourami
  • 10-15 Glowlight rasbora
  • 8-10 Cherry Shrimp

Key Factors To Consider While Keeping Fish In 20-Gallon Tank

rainbow kribs in 20 gallon tank
Image Credit: JuanCarlosPalauDiaz, Pixabay

There are a few factors that you need to consider before deciding how many fish to put in your 20-gallon tank.

  • Water Parameter: Fish can be very sensitive to changes in pH, temperature, and water hardness, so it is important to make sure that all of the fish you choose share the same requirements.
  • Temperature: Temperature is critical for both tropical and cold-water fish. Make sure that your aquarium has a heater, and monitor the temperature with a good-quality aquarium thermometer.
  • Water Quality: The amount of waste produced by fish increases with the number of fish in the tank. It is important to keep this in mind when stocking your aquarium, as poor water quality can lead to health problems for your fish.
  • Plants: A planted 20-gallon tank will provide better oxygenation for twenty 1-inch fish than a bare tank. This is due to the oxygen produced by the plants in the tank.
  • The type of fish you want to keep: Some fish are more active than others and will require more space.
  • The Adult size of the fish: Some fish grow to be very large, while others stay quite small.

You must consider the fish compatibility when stocking your aquarium with multiple fish species. Some fish are more aggressive and may not do well with others. It is important to research the fish you are considering to be sure they will be compatible with the other fish in your aquarium.

Frequently Asked Questions

Can 1 male and 1 female betta fish live together?

Male betta fish cannot live together because they will fight each other. Male betta fish are aggressive and territorial towards other fish, including female betta fish.

Can I keep 2 axolotls in a 20-gallon tank?

No, two axolotls will not be able to live together in a 20-gallon tank. Axolotls are solitary creatures and they need a lot of space to themselves, so a 55-75 gallon tank is necessary to house two axolotls.


A 20-gallon tank is great for various fish, including bettas, cichlids, and rainbows kribs. It’s important to choose your fish carefully so they all have the space they need to thrive in the tank. With a little knowledge, you can create an aquarium perfect for your needs and interests.

Featured image credit: Tuan86, Pixabay

Related article: Best stands for 20 gallon tanks


Jason Matthews

My name is Jason Matthews, and welcome to my website. When other kids were bragging about how their dog could sit and roll over, I was bragging about my latest Betta Fish and the cool sea castle I just added to his aquarium. 

Jason aquariume

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