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Best Fish for 20-Gallon Tank (Including Combination Ideas)

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by Jason Matthews



Fish aquariums come in all shapes and sizes, but the 20-gallon tank is by far the most popular among fish keepers. This is because its size is enough to be exciting and manageable for beginners.

The beauty of this tank is that it’s small enough to place nearly anywhere but large enough to house a wide variety of fish. This tank also affords you the opportunity to get creative with decor and incorporate some greenery for the well-being of your new aquarium friends.

Are you interested in having a 20-gallon fish tank, but you’re not sure which fish to put in your aquarium? If so, then you’ve come to the right place! Deciding on the best fish for your 20-gallon tank can be tricky since there are many things to consider. 

Before adding it to the tank, you need to know how large the fish will grow and thoroughly understand its needs. This blog post will discuss some of the best fish options for your 20-gallon aquarium. So, keep reading to learn more!

8 Best Fish For Your 20-Gallon Aquarium

After using the knowledge of years and scouring the Internet for hours, we’ve put together a list of the best fish you could hope to find for your 20-gallon fish tank.

Rosy Tetras (Boraras brigittae)

Rosy Tetras (Boraras brigittae)
Image: Besjunior, Canva

Rosy tetras are a colorful addition to any aquarium. They have three black vertical bars on their sides and boast a beautiful rosy hue. Rosy tetras are hardy fish that adapt well to aquarium life, making them suitable for beginners.

The rosy tetra measures an average length of 2 inches and generally lives to be 2 years old. They are friendly with many other fish species but have been known to chase the occasional rosy tetra while swimming either horizontally or at close range.

To provide the best home for your rosy tetras, it is recommended that you keep no more than six in a 20-gallon tank. The temperature of their water should be maintained at 75°F to 82°F.

The rosy tetra is a mostly peaceful fish but will protect itself when necessary. If another rosy tetra tries to bully it or if it gets startled, the rosy tetra may become aggressive and flare its fins at the other fish.

Goldfish  (Carassius auratus)

Photo by bankmoophoto, Canva

Goldfish are popular pets among households that have a garden pond or aquarium because they don’t require much maintenance and their colors are bright. They’re a part of the family Cyprinidae and are usually gold or orange-colored.

Depending on the type and age, a goldfish can grow up to 6 inches in length. If well taken care of, a goldfish has a lifespan of 20 years.

Goldfish are relatively peaceful creatures that prefer living in water conditions of 68°F to 74°F. You can comfortably keep two goldfish in a 20-gallon tank.

Guppies (Poecilia reticulata)

guppy fish
Image: underworld1, Depositphotos

Guppies are a great type of fish to have in a 20-gallon tank and don’t need much care, making them perfect for beginner and experienced aquarists. Guppies are popular among fishkeepers due to their stunning colors, ease of care, and peaceful nature. Guppies are very colorful; therefore, they are often called rainbow fish.

The main reason guppy fins look amazing is that they don’t have bigger heights. The maximum height for a male or female guppy is only 2 inches. However, of the two genders, females tend to reach heights close to 2.5 inches while males rarely surpass 2 inches.

Did you know that guppies can survive in sunlight? This means you could put your fish tank near a window where it would get direct sunlight. Just be sure to monitor the water temperature in the tank, which should be 72-78°F.

Zebra Danios (Danio rerio)

Photo by neryxcom, Canva

Danios are freshwater fish that belong to the Danionidae family. They provide a beautiful look to your aquarium tank and come in many colors and types. Zebra danios are one of the more common varieties because they tend to be peaceful and social creatures. In addition, zebra danios have vertical stripes all over their bodies which makes them stand out from other fishes.

Zebra danios have an approximate lifespan of 2 years. maximum length They reach is 1.5 to 2 inches, and they prefer to live in aquariums where the water temperature does not exceed 27 degrees Celsius (80°F). For their optimal growth, zebra danios need a continuous source of light for at least 12 hours every day.

Danios are very active fish and like to swim freely, so a 20-gallon tank is the best choice to keep them. Zebra danios are easy to care for and don’t require much attention. They’re also great companions for other active fish.

Molly Fish (Poecilia sphenops)

molly fish
Image: darkocv, Depositphotos

Our next choice would be a molly fish. They have an attractive, unique appearance with a triangular head and flat body. The fan-shaped tail is what really sets them apart from other fish though.

Mollies do best in 20-gallon tanks at temperatures 72-78°F and are very easy to keep due to their peaceful and friendly nature. You wouldn’t have any difficulties looking after a molly.

The size of mollies can differ based on the type of species. For example, some molly fish could grow up to 4-4.5 inches in height. Additionally, it’s interesting to note that mollies can change their sizes depending on the size of their tank environments.

Platy (Xiphophorus macalatus)

Photo by hadot, Canva

Platies are an easy fish to take care of, so they’re a good choice for someone who is just starting out with a 20-gallon tank. Platies can also be successfully kept by more experienced aquarium owners.

Small and beautiful, platies are popular aquarium fish known for their variety of colors and tail shapes. One species, the Mickey Mouse Platy, gets its name from its unique shape.

Platies reach a maximum height of 3 inches, while the dwarf platy only grows to be 1 inch tall at most. If you have a 20-gallon aquarium, it can hold up to eight platies comfortably. It’s important to keep them in an appropriate ratio though. We recommend having at least two female platies for every male platy in the tank.

Swordtails (Xiphophorus hellerii)

swordtail fish
Image: darkocv, Depositphotos

The swordtail fish is a beautiful and popular choice for aquariums. They get their name from their long, “sword-like” tailfeathers which come in many colors and patterns. Best of all, they are easy to care for, even in small spaces like a 20-gallon tank.

Swordtails will do well in many environments, though they lean toward more light and plant life. If there is Swordtail Fry, these plants can offer them some hiding places from any predators that might be present. As active bottom feeders, swordtails offer a great addition to any aquarium!

When kept in captivity, swordtails prefer living in slightly acidic water with a pH level of 6-7 and a temperature of 72-78°F. On average, mature swordtails are 3-5 inches in length. It’s recommended that you should keep at least six swordtails together, and they will breed prolifically given the chance.

The Cory Catfish (Corydoras)

Two small spotted Cory catfish side by side
Image: Trimitrius, Depositphotos

Cory Catfish is a stunning freshwater option for 20-gallon fish tanks. This group of small fishes has bodies armored by bony plates and spines that can be quite sharp and sometimes venomous.

Cory Catfish are peaceful, armored fish that have short faces, flat undersides, and pectoral fins. They use their dorsal fin to point up and stay hidden when they feel threatened. Cory Catfish are easy to care for because they won’t attack other tank mates.

They can grow to be about 4 inches tall and have a lifespan of 5 years if you take proper care of them. They prefer a tank temperature between 21-26 degrees Celsius (70-78 Fahrenheit).

Related: What filter is the best for a 20 gallon tank?

Your aquarium will stay healthy and balanced by stocking your 20-gallon tank with different fish. Having one or two larger “Centerpiece” fish is generally a good idea. A school of smaller fishes can occupy the rest of the tank along with a few calm bottom dwellers such as shrimp or catfish.

It is best to choose peaceful fish if you want different species living together harmoniously. For example, the pea puffer is an unsuitable tank mate in a small aquarium because they are highly territorial or aggressive, despite only being 1.4 inches long.

So, if you’re stocking a 20-gallon tank, here are some popular fish combos to consider:

Combo #1

Combo #2

Combo #3

  • 6 Rosy Barbs
  • 1 male + 5 female Mollies
  • 6 Pygmy Corydoras.

Combo #4

Combo #5

  • 6 Endler’s Livebearers (more females than males, M-F 1:2 ratio)
  • 6-8 Tetras
  • 6 Pygmy Corydoras

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

Final Thoughts

Before buying any fish for your 20-gallon tank, consider how big the fish will be when it’s fully grown. That’s because most pet stores sell juvenile fish that haven’t reached maturity. In addition, research the species’ behavior and compatibility with other kinds of fish before making a purchase, as some are peaceful while others are aggressive.

Some fish species prefer to lounge around, while others need more room to swim, so you must do your research before picking the right fish for your tank. Different fish also have different water preferences in terms of temperature and hardness,  so be sure that the conditions in your tank are suitable for the type of fish you want.

So, you now understand which fish work best in a 20-gallon tank. Just go for it!

Related article: Best stands for a 20 gallon tank

Featured image: Chait Goli, Pexels


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. 

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