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What Are The Best Fish For A 20-Gallon Tank? Here Are 10 Great Options

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The Best Fish For A 20-Gallon Tank

Whether you’re a beginner in the fishkeeping scene or just want to add another aquarium to your living space, a few suggestions on what are the best fish for a 20-gallon tank would come in handy.

A 20-gallon tank is usually the most popular tank size because it’s not bulky, and you can keep it in your bedroom or dining room. Furthermore, it offers enough space for you to choose from a wide variety of fish.

From experience, the most difficult part is deciding on the species of fish you want to keep in your aquarium. They should have resembling behaviors, almost the same water temperature demands, and also get along.

However, the most important part is that they’re the right size for a 20-gallon tank. So, let’s see!

The Harlequin Rasbora (Trigonostigma heteromorpha)

Harlequin Rasbora, rasbora heteromorpha

The Harlequin Rasbora is a cute, small fish that’s been a constant favorite among aquarists since its introduction in the early 1900s. And why wouldn’t you love it? Is it one of the best fish for a 20-gallon tank? Let’s take a closer look!

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivorous
  • Water temperature: 22 °C to 27 °C (72 °F to 81 °F)
  • Size: Up to 2 inches
  • Family: Cyprinidae
  • Lifespan: 5-8 Years

Harlequin Rasbora Appearance

When we said they’re little, we meant it. These Rasboras don’t really grow larger than 2 inches. Their bodies are usually silver, with tints of orange and dark orange on the fins. Their bodies’ mid-section is tall, but it narrows down, approaching the mouth and the forked caudal fin.

The orange color can vary in intensity depending on their breeding and the conditions in the water tank. Since they’re so small, you can fit quite a few in one tank, and together they look spectacular.

Males and females are almost completely alike to the eye of the usual enthusiast. However, if you study them a little bit closer, you’ll be able to see that males have larger darker patches on their bodies and a more rounded section near the anal fin.

Harlequin Rasbora Temperament

The Harlequin Rasboras are very peaceful little fish. They don’t fight with their tank mates. On the contrary, they could fall victim to other fish’ rowdy behavior and become stressed.

They will just spend most of the time in the middle layers of the water and only darting to the surface in emergencies when they might get threatened by other fish. They like caves and plants and other types of water tank accessories.

Harlequin Rasbora Tank Mates

For this kind of peaceful small creatures that would stand out of everyone’s way, make sure you avoid any way-larger species like cichlids or even Boisterous. Don’t put Harlequin Rasboras in the same tank with overactive fish known for harassing other mates, like Clown Loaches or Bettas.

You’ve got to go for the small, equally-peaceful fish such as Cherry Barbs, Dwarf Gourami, Guppies, Mollies, Platies, Tetras, Corydoras Catfish, even Danios.  You can also keep snails or shrimp in the aquarium as they go along well with Harlequin Rasboras (by well, we mean they won’t even notice each other).

The Otocinclus (Otocinclus)

Closeup of an otocinclus in planted aquarium

Otocinclus is a genus of catfish native to South America, usually nicknamed “dwarf suckers” or simply “otos”.

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Herbivore
  • Water temperature: 22 °C to 28 °C (72 °F to 82 °F)
  • Size: 1-2 inches
  • Family: Loricariidae
  • Lifespan: 3-5 Years

Otocinclus Appearance

In general, the Otocinclus has a cylindrical body that narrows towards the mouth and caudal fin. They have quite strong mouths for their size, and they use them to latch onto surfaces.

Although generally a catfish, the Otocinclus has some armor plating across its body to help protect against other rowdy species or rough surfaces.

The females are somewhat larger and broader than males, but it is quite difficult to spot the difference. As for colors and patterns, they vastly depend on the type of Otocinclus species you choose, and there are a lot to choose from.

Otocinclus Temperament

They’ll be “the nice guys” in your aquarium. They stay out of the way of other species. When they are scared, they are usually rushing from one part of the tank to the other, and they’re quite fast. So, catching them in a net might prove a challenge.

They like to roam at the bottom of the tank or lay on the surfaces of the decorations, plants, and tank walls. They tend to stick together as a group.

Otocinclus Tank Mates

First thing first, they’re small, so avoid any large and aggressive fish. As a rule of thumb, stay away from any aggressive species with a mouth big enough to swallow an Oto.

Some popular tank mates for Otos are Zebra Loaches, Guppies, Cherry Barbs, Harlequin Rasbora, even Angelfish. Shrimps and snails are also good tank mates for them.

The Guppy (Poecilia reticulata)

Guppy

The guppies, aka “millionfish” or “rainbow fish,” are some of the best-known tropical fish on the planet, and it’s not difficult to see why.

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivores
  • Water temperature: 22 °C to 28 °C (72 °F to 82 °F)
  • Size: 6- 2.4 inches
  • Family: Poeciliidae
  • Lifespan: Up to 2 years

Guppy Appearance

To pin down a certain guppy appearance is almost impossible, as they come in so many colors and sizes that it will take a while to categorize all of them.

Generally, the female Guppies are grey, and the males have all sorts of colorful stripes, spots, or sprinkles. There are many aquarium varieties you can choose from, and that’s completely up to you. Even as we speak, they might breed new varieties.

You can tell a Guppy male from a female just by looking at them, as males are usually smaller than females.

Guppy Temperament

Well, they’re peace lovers and love to stick together in large groups. They are quite active, and you’ll see them move around all the time. The males would chase the females, trying to impress.

They are so active all day long that you’ll immediately sense when they’re stressed or scared of something – that’s when they’ll hide and lay low.

Guppy Tank Mates

Usually, it’s more guppies, as many as possible. Besides, they’re so many species of them in so many different colors that you might not need any other tank mates other than other guppies.

However, try to keep a ratio of 2:1 males and females. Still, if you want a lot more diversity, they could be good tank mates with Platies, Gouramis, Mollies, or Corydoras. Do not get them any other aggressive fish species.

The Zebrafish (Danio rerio)

Zebrafish

This freshwater fish is native to South Asia, notable for its regenerative abilities, and also quite popular for aquarium enthusiasts who might know it as “zebra danio”.

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivores
  • Water temperature: 22 °C to 28 °C (72 °F to 82 °F)
  • Size: 2 inches
  • Family: Cyprinidae
  • Lifespan: 5- 5.5 years

Zebrafish Appearance

In the wild, the Zebrafish is silver-gold and presents about 5 stripes from the head to the caudal fin. The males are more golden-colored as compared to the more silvery-white females.

However, you will find them in many colors for the aquarium and with a longfin appearance, as they have been selectively bred to appeal to aquarium enthusiasts.

Zebrafish Temperament

Zebrafish are very social and peaceful, sometimes living in shoals, with an established hierarchical system. So, make sure you don’t leave them alone in the aquarium, as they could become stressed and choose to hide all the time.

Also, keep in mind that they might sometimes get aggressive, especially when paired with slow-moving long-finned fish species.

Zebrafish Tank Mates

First, avoid long-finned fish as the Zebrafish love to nip at the tail fins of these types of fish. Of course, avoid any other type of predatory fish like Redtail Catfish.

Other than that, you can pair them with Flying Barb, Emerald Pufferfish, Ember Tetra, Celestial Pearl Danio, Rosy Barbs, Bronze Cory, etc.

They also love aquatic invertebrates, like Zebra snails or even frogs such as the African Dwarf Frog or shrimp.  

The Dwarf Gourami (Trichogaster lalius)

red dwarf gourami in the water tank close up

Native to Pakistan, India, or Bangladesh, the dwarf gourami loves natural slow-moving streams, lakes, or rivulets with plenty of vegetation.

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivores
  • Water temperature: 22 °C to 28 °C (72 °F to 82 °F)
  • Size: 4 – 4.5 inches
  • Family: Osphronemidae
  • Lifespan: Up to 5 years

Dwarf Gourami Appearance

Dwarf Gouramis usually have unique colors, most of them a result of systematic breeding for aquariums. They could be blue with reddish-brown lines across their sides, bright red with an orange gradient, or neon blue with red stripes.

They have a narrow, compressed-on-the-sides body, with large and round fins. Their anal and dorsal fins are merged, while their ventral fins have a filiform shape and act as a sensory organ.

Dwarf Gourami Temperament

 They are quite peaceful and tolerant species. However, they do not like loud noises. They tend to hide in hard-to-access places when they are stressed. So, keep an eye out for them, not to get trapped.

They need quite a long period of adapting to a new water tank environment, so it’s normal to see them standing still in one place at first.

Dwarf Gourami Tank Mates

Try to pair them with species that love to swim in the bottom or middle level of the tank so they won’t have territorial disputes in the aquarium.

The ideal bottom dwellers are other Gouramis, Mollies, Loaches, Tetras, Otocinclus Catfish, Platies, or Rasboras. They also get along well with Mystery Snails or Amano Shrimps.

They don’t like very active and fast fish like barbs, as they would have to compete with them for food.

The Goldfish (Carassius auratus)

Goldfish

One of the best fish for a 20-gallon tank and one of the most common aquarium fish, this small member of the carp family is native to East Asia.

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivores
  • Water temperature: 20 °C to 24 °C (68 °F to 74 °F)
  • Size: 6 – 12 inches
  • Family: Cyprinidae
  • Lifespan: 8-10+ years

Goldfish Appearance

Although the name says it all about this species’s appearance, years of selective breeding have created a couple of color variations; some of them don’t even have anything else to do with “gold”.

The common goldfish comes in a variety of red, orange, gold, white, black, or yellow colors, but then you have a lot of other species.

Goldfish Temperament

Goldfish are an overall friendly species, with some displays of schooling behavior. They do not hurt each other; the males do not hurt the females during breeding either.

They might compete for food, and if you have different varieties of goldfish, the common ones might eat all the food before the fancy varieties get some. So, fancier varieties might have to suffer in this pairing.

Goldfish Tank Mates

Make sure you pair goldfish varieties with similar body types and swim characteristics for the feeding reason mentioned above. You can pair them with rosy barbs, Zebra Danios, Banded Corydoras, or Platies.

Goldfish usually eat live plants, so placing them in a planted aquarium might prove problematic for the plants. Cryptocoryne and Anubias might survive with goldfish in the same aquarium, but they’re quite sensitive and require special attention from you.

The Celestial Pearl Danio (Danio margaritus)

Celestial Pearl Danio

This is the Galaxy Rasbora you might have heard of from pet shops. It has been discovered late in 2006 and quickly made it to the aquarium trade for its small size and bright colors. They’re some of the best fish for a 20-gallon tank.

  • Care Level: Medium
  • Temperament: Peaceful
  • Diet: Omnivores
  • Water temperature: 23 °C to 26 °C (73 °F to 79 °F)
  • Size: Up to 1 inch
  • Family: Cyprinidae
  • Lifespan: 3-5 Years

The Celestial Pearl Danio Appearance

They’re colorful balls of energy. They have white-spotted bodies with red or orange fins and will immediately stand out in the aquarium.

The two parallel black lines on their orange fins are making them stand out from the rest. They have sexual dimorphism, which means the females and males look quite different from one another.

The males are thinner and have a more vibrant color, while the females are more round in shape, with a less striking color.  When the males start courting, their stomachs will gain an additional deep red coloration, and they could even put up serious fights for their future mates.

Celestial Pearl Danio Temperament

They live in groups and are quite active. The males are almost always courting the females and quarreling with rivals. You don’t have to worry too much about them fighting, though; it is mostly a ritual.

Celestial Pearl Danio Tank Mates

Demure and peaceful in nature, this fish species can happily live together with Mollies, Killifish, Tetras, Guppies. They have special compatibility with Neon Tetras. Nevertheless, they like peaceful communities, so you should get them together with other species that follow the same behavioral pattern.

The Neon Tetra (Paracheirodon innesi)

Neon tetra fish with aquatic plant in aquarium

Native to blackwater and clearwater streams in the Amazon basin, the Neon Tetra is an insanely brightly-colored fish that would perfectly fit in your 20-gallon tank.

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivores
  • Water temperature: 21 °C to 28 °C (70 °F to 81 °F)
  • Size: Up to 1.5 inch
  • Family: Characidae
  • Lifespan: 8 Years

Neon Tetra Appearance

They boast fantastic coloring. You cannot help but wonder at that turquoise blue line stretching from the eyes to the adipose fin. Then there’s that red stripe running from the middle of their body to the caudal fin.

They let out an iridescent rainbow effect that enables them to signal each other in the murky dark waters of their natural habitat.

Neon Tetra Temperament

They are not aggressive and love to live in large communities. They’re as peaceful as they are beautiful, and they could make for the best fish for a 20-gallon tank.

Neon Tetra Tank Mates

A school of peaceful neon tetra is more than enough for a 20-gallon tank. If you wish for more diversity, you could pair them with other peaceful bottom dwellers like Dwarf Cichlids, Barbs, Small catfish, or Gouramis.

The Cory Catfish (Corydoras)

Corydoras fish on the bottom and in the aquarium.

This genus of freshwater catfish contains small fish species that are protected by body armor and sharp, sometimes venomous spines.

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivores
  • Water temperature: 21 °C to 26 °C (70 °F to 78 °F)
  • Size: 75-4 Inches
  • Family: Callichthyidae
  • Lifespan: 5 Years

Cory Catfish Appearance

In general, Cory Catfish have a short face and flat underside, some pectoral fins to feel the surfaces, and a dorsal fin that points up. They have an armored body to protect themselves against predators.

The three pairs of barbels on their face help them detect the food, while their wide eyes make them look like they’re always wondering what’s going on, and that’s quite cute.

As for colors, there are a lot of those going around, for each species, from shades of brown to emerald or even albino.

Cory Catfish Temperament

Cory Catfish dwell on the bottom of the tank and usually rest during the day. In the evening, they will shoal with other fish. Sometimes they might spring to the surface to get some air.

They’re peaceful, won’t attack other tank mates, and hide when they feel in danger.

Cory Catfish Tank Mates

Cory Catfish live with neon tetras or phantom tetras in their natural habitat. So, you can pair them like that in your aquarium as well.

They could also get along well with guppies, mollies, or swordtails.

The Molly Fish (Poecilia sphenops)

Colorful aquarium Molly fishes. They inhabit fresh water streams and coastal brackish and marine waters of Mexico. The wild-type fishes are dull, silvery in color.

The Mollies are some of the most popular aquarium fish; they can grow rapidly and populate freshwater streams. They are some of the best fish for a 20-gallon tank.

  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivores
  • Water temperature: 23 °C to 26 °C (72 °F to 78 °F)
  • Size: Up to 4.5 inches
  • Family: Poeciliidae
  • Lifespan: Up to 5 Years

The Molly Fish Appearance

They come in many varieties, with a lot of different colors, patterns, shapes, and even sizes.

A common molly would have a flat body that narrows towards the mouth. Its caudal fin would be large and transparent in color. It could boast a dorsal fin raised as a fan or flat against the body.

Usually, females are larger than males, and they’re quite easy to differentiate.

Molly Fish Temperament

Mollies are peaceful until they are crowded or surrounded by some aggressive tank mates. In that case, they turn aggressive.

Apart from that, they enjoy schooling together and could even form shoals. They tend to have quite different personalities, and if you love watching them all day, you will find those temperamental differences at some point.

Molly Fish Tank Mates

Some suitable tank mates for mollies are Cherry Barbs, Corydoras Catfish, Dwarf Gourami, Zebra Loaches, or Harlequin Rasbora.

Just avoid any species that are too large or aggressive.

Conclusion – The Best Fish For A 20 Gallon Tank

You have a lot of choices, and this fact might make your choice a lot more difficult. Nevertheless, buying a 20-gallon tank and looking for your new friends to populate it can also be one of the most pleasurable activities, especially if you are an aquarium hobbyist. Now that you know which are the best fish for a 20-gallon tank, just do it!