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Best Fish For A 20-Gallon Tank And How Many Can You Fit?

A 20-gallon tank is ideal for a beginner to the hobby and offers lots of exciting options. An aquarium of that size enables you to keep a range of freshwater fish, living plants, and invertebrates, too.

Before you begin your fish keeping journey, you’ll need to know what fish to choose for your aquarium and how many fish can live in a 20-gallon tank.

In this guide, we introduce you to 43 of the best fish for a 20-gallon tank, and much more!

Table of Contents

How Many Fish for a 20-Gallon Tank?

No doubt you will be keen to fill your fish tank with lots of exciting and beautiful fish. So, how many fish can you keep in a 20-gallon tank?

There’s a generally accepted guideline for stocking any aquarium, and that’s to allow one inch of fish per one gallon of water in the tank. However, there are a few other things to consider when stocking your tank, which we’ll discuss later in this guide. 

The most important thing to know is that you shouldn’t overstock your tank. Squeezing too many fish into the tank leads to overcrowding, stress, and poor water conditions, none of which is good for your pets’ health and longevity.

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

Rosaline Shark (Sahyadria denisonii) - Care Guide

Some hobbyists like to keep one feature fish in a 20-gallon beautifully aquascaped tank. So, what’s the biggest fish you can put in a 20-gallon tank?

Although a 20-gallon tank does offer a fair amount of space, whatever species you choose, the fish must have plenty of room to swim and turn around. For example, extremely active, large fish such as Roseline Sharks need a tank that allows them adequate space to shoal and moves around freely.

Ideally, you want to keep fish that grow no bigger than six inches in a 20-gallon tank, applying the one inch per gallon rule for multiple specimens.

What to Consider When Choosing Fish For a 20-gallon Tank

Adult Size of the Fish

Most of the fish sold in fish stores are juveniles that still have a fair amount of growing to do before they reach adult size. 

Striped Raphael Catfish (Platydoras armatulus)

For example, some catfish species are sold as cute little creatures no more than a couple of inches long. However, those babies can quickly grow to measure up to a foot long! So, before buying a fish, do plenty of research to find out how big that fish will get.

Behavior and Compatibility

Another vital consideration is the behavior of the fish species you choose.

In nature, many small fish are schooling species that adopt the strategy of safety in numbers. For that reason, those species must be kept in shoals of at least six individuals to avoid the fish becoming stressed and falling prey to fish diseases.

Harlequin Rasbora, rasbora heteromorpha, Aquarium Fishes

In contrast, some fish are aggressive and do best when kept alone or with a few carefully chosen tank mates for company. Other species are extremely active and need lots of open water swimming space to be happy.

You must do your research before you buy any fish to make sure that the species you choose are compatible. Click this link to find out what fish go together and what species don’t get along.

Water Parameters

Each species of fish has different preferences when it comes to water parameters. Giving your fish the right water conditions is essential if you want your fish to stay healthy and do well.

So, make sure that the fish species you choose all share the same pH, temperature, and water hardness requirements.

Pouring treatment to water

Temperature is critical. Both tropical and coldwater fish can suffer from temperature shock if the water temperature is too warm or too cold for them. Make sure that your aquarium has a heater, and monitor the temperature with a good-quality aquarium thermometer. And never try to mix coldwater fish with tropical species.

Stocking Ideas for Your 20-Gallon Fish Tank

The choice of fish for your 20-gallon tank is mind-boggling! So, here are a couple of ideas for a harmonious community tank setup to get you started:

Livebearer Community Tank

  • 3 x Guppies
  • 2 x Platies
  • 2 x Mollies

Tetra Community Tank

danio rerio fish and neon corals close-up
  • 5 x Neon tetras
  • 5 x Glowlight tetras
  • 3 x Black Phantom tetras
  • 3 x Cherry shrimp

Shrimp are often overlooked as additions to a community tank. That’s a shame, as these industrious little critters make a colorful and exciting addition. Some species eat algae, too, helping to keep your tank clean.

What About Plants?

Live plants are highly beneficial for the ecosystem in your fish tank. As well as adding aesthetic beauty to your aquarium, living plants extract harmful nitrates and CO2 from the water for use as fertilizer. Plants also help to oxygenate the tank as they photosynthesize.

Aquarium Plants

Many hardy aquatic plants are simple to care for and make the ideal choice for beginners. Just double-check that the fish species you’ve chosen are plant-safe.

A few excellent beginner plants that grow well in medium to low light conditions include:

  • Anubias
  • Amazon Sword
  • Vallisneria
  • Java moss
  • Marimo moss balls
  • Hornwort

43 Best Fish Species for a 20-Gallon Tank

Here are 43 fish and invertebrate species that you can keep in an aquarium of 20 gallons or larger.

Tetra

1. Cardinal Tetra (Paracheirodon axelrodi)

Cardinal Tetra
  • Family: Characidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73o to 81o F
  • Max. Size: 2 inches
  • Lifespan: Up to 2 years

2. Ember Tetra (Hyphessobrycon amandae)

The ember tetra (Hyphessobrycon amandae) is a freshwater fish of the characin family (family Characidae) of order Characiformes.
Image Source: wikimedia.org
  • Family: Characidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 68o to 82o F
  • Max. Size: 2 inches
  • Lifespan: Up to 2 years

3. Rosy Tetra (Hyphessobrycon rosaceus)

Rosy Tetra (Hyphessobrycon rosaceus)
Source : instagram.com
  • Family: Characidae
  • Care Level: Intermediate
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 75o to 82o F
  • Max. Size: 1.6 inches
  • Lifespan: Up to 2 years

4. Flame Tetra (Hyphessobrycon flammeus)

Flame Tetra (Hyphessobrycon flammeus)
Learn more : flickr.com
  • Family: Characidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73o to 82o F
  • Max. Size: 1.6 inches
  • Lifespan: Up to 2 years

5. Glowlight Tetra (Hemigrammus erythrozonus)

Clsoseup oa a glowlight tetra in aquaruim
  • Family: Characidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 74o to 82o F
  • Max. Size: 1.6 inches
  • Lifespan: Up to 2 years

Rasbora

6. Lambchop Rasbora (Trigonostigma espei)

Lambchop Rasbora (Trigonostigma espei)
Image Source : wikimedia.org
  • Family: Cyprinidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73o to 80o F
  • Max. Size: 1.2 inches
  • Lifespan: 3 to 5 years

7. Chili Rasbora (Boraras brigittae)

Mosquito Rasbora (Boraras brigittae) Beautiful Endemic Ornamental fish from Borneo
  • Family: Cyprinidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 68o to 82.4o F
  • Max. Size: 0.8 inches
  • Lifespan: 3 to 4 years

8. Least Rasbora (Boraras urophthalmoides)

Least Rasbora (Boraras urophthalmoides)
  • Family: Cyprinidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73.4o to 82.4o F
  • Max. Size: 1.6 inches
  • Lifespan: 4 to 8 years

9. Harlequin Rasbora (Trigonostigma heteromorpha)

HARLEQUIN RASBORA
Learn more: flickr.com
  • Family: Cyprinidae
  • Care Level: Intermediate
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73o to 82o F
  • Max. Size: 2 inches
  • Lifespan: 5 to 6 years

10. Red Dwarf Rasbora (Microrasbora rubescens)

Red Dwarf Rasbora (Microrasbora rubescens)
Source : instagram.com
  • Family: Cyprinidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 68o to 75.2o F
  • Max. Size: 0.8 inches
  • Lifespan: 4 to 8 years

11. Dwarf Rasbora (Boraras maculatus)

Close up selective focus of Dwarf rasbora or Boraras maculatus
  • Family: Cyprinidae
  • Care Level: Intermediate
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 74o to 79o F
  • Max. Size: 1 inch
  • Lifespan: 3 to 5 years

12. Galaxy Rasbora (Microrasboras galaxy)

Galaxy Rasbora (Microrasboras galaxy)
Image Source : wikimedia.org
  • Family: Cyprinidae
  • Care Level: Intermediate
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 70o to 75o F
  • Max. Size: 1 inch
  • Lifespan: 3 to 5 years

Platies

13. Parrot Platy (Xiphophorus hybrid)

Parrot Platy (Xiphophorus hybrid)
Source : instagram.com
  • Family: Poeciliidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 72o to 75o F
  • Max. Size: 3 inches
  • Lifespan: 3 to 5 years

14. Red Platy (Xiphophorus maculatus)

Wagtail Platy Fish, xiphophorus maculatus
  • Family: Poeciliidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 64o to 77o F
  • Max. Size: 2 inches
  • Lifespan: 4 years

15. Coral Platy (Xiphophorus maculatus)

Coral Platy (Xiphophorus maculatus)
Image Source : wikimedia.org
  • Family: Poeciliidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 64o to 77o F
  • Max. Size: 1 inch
  • Lifespan: 4 years

Guppies

16. Endler’s Livebearers (Poecilia wingei)

Brightly colored Endler’s livebearer fishes.
Image Source : wikimedia.org
  • Family: Poeciliidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 78o to 80o F
  • Max. Size: 1 inch
  • Lifespan: 2 to 3 years

17. Fancy Guppies (Poecilia reticulata)

Half-Black Blue Guppy
Source : tankfacts.com
  • Family: Poeciliidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 68o to 82o F
  • Max. Size: 2.4 inches
  • Lifespan: 1 to 3 years

Cichlids

18. Angel Ram Cichlids (Mikrogeophagus ramirezi)

Angel Ram Cichlids (Mikrogeophagus ramirezi)
Source : instagram.com
  • Family: Cichlidae
  • Care Level: Difficult
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73o to 80o F
  • Max. Size: 3 inches
  • Lifespan: 3 years

19. Electric Blue Ram (Mikrogeophagus ramirezi)

Close up image of an Electric Blue Ram in a blackwater aquarium
  • Family: Cichlidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 78o to 86o F
  • Max. Size: 3 inches
  • Lifespan: 2 to 4 years

20. Umbrella Cichlid (Apistogramma Borelli Opal)

Umbrella Cichlid (Apistogramma Borelli Opal)
  • Family: Cichlidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 72o to 78o F
  • Max. Size: 2.6 inches
  • Lifespan: 2 to 3 years

21. Yellow Dwarf Cichlid (Apistogramma borellii)

  • Family: Cichlidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 78o to 84o F
  • Max. Size: 2.8 inches
  • Lifespan: 2 to 3 years

22. Julies (Julidochromis)

Julies (Julidochromis)
Image Source : wikimedia.org
  • Family: Cichlidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73o to 77o F
  • Max. Size: depends on species
  • Lifespan: 5 to 8 years

23. Apistogramma (Xiphophorus hellerii)

Apistogramma (Xiphophorus hellerii)
Learn more : flickr.com
  • Family: Cichlidae
  • Care Level: Intermediate
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 72o to 86o F
  • Max. Size: 3 inches
  • Lifespan: 5 to 10 years

Gourami 

24. Honey Gourami (Trichogaster chuna)

Honey gourami Trichogaster chuna tropical aquarium fish in fish tank. aquaria concept
  • Family: Osphronemidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 72o to 82o F
  • Max. Size: 3.2 inches
  • Lifespan: 4 to 8 years

25. Dwarf Gourami (Trichogaster lalius)

This image shows a Dwarf Gourami female (Colisa lalia).
Image Source: wikimedia.org
  • Family: Osphronemidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 72o to 82o F
  • Max. Size: 3.5 inches
  • Lifespan: 5 to 7 years 

26. Red Dwarf Gourami (Trichogaster lalius)

Dwarf gourami on a blurred background (Trichogaster lalius)
  • Family: Osphronemidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 72o to 82o F
  • Max. Size: 3.5 inches
  • Lifespan: 5 to 7 years

27. Sparkling Gourami (Trichogaster pumila)

Sparkling Gourami (Trichogaster pumila)
  • Family: Osphronemidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 70o to 82o F
  • Max. Size: 1.5 inches
  • Lifespan: 5 years

Corydoras Catfish

28. Panda Corydoras Catfish (Corydoras panda)

Panda Cory
  • Family: Callichthyidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 68o to 77o F
  • Max. Size: 2 inches
  • Lifespan: 10 years

29. Albino Corydoras Catfish (Corydoras paleatus)

Green or Bronze Corydoras (Corydoras Aeneus)
Image Source : wikimedia.org
  • Family: Callichthyidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 68o to 77o F
  • Max. Size: 2 inches
  • Lifespan: 10 years

30. Skunk Corydoras Catfish (Corydoras arcuatus)

Skunk Corydoras Catfish (Corydoras arcuatus)
  • Family: Callichthyidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 68o to 77o F
  • Max. Size: 2 inches
  • Lifespan: 10 years

Shrimp 

31. Red Fire Shrimp (Lysmata debelius)

Cherry Shrimp (Neocaridina davidi)
  • Family: Hippolytidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore, algae eater
  • Water Conditions: Saltwater, 72o to 78o F
  • Max. Size: 2 inches
  • Lifespan: 1 year

32. Tiger Shrimp (Caridina serrata)

Tiger Shrimp (Caridina serrata)
Image Source : wikimedia.org
  • Family: Penaeidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore, algae eater
  • Water Conditions: Freshwater, 68o to 74o F
  • Max. Size: 1.25 inches
  • Lifespan: 1 to 2 years

33. Amano Shrimp (Caridina multidentata)

Amano Shrimp (Caridina multidentate)
  • Family: Atyidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore, algae eater
  • Water Conditions: Freshwater, 64o to 80o F
  • Max. Size: 2 inches
  • Lifespan: 2 to 5 years

Miscellaneous

34. Betta Fish (Betta splendens)

fancy crowntail betta fish
Learn more: flickr.com
  • Family: Osphronemidae
  • Care Level: Easy
  • Temperament: Semi-aggressive – males cannot be kept together
  • Diet: Omnivore
  • Water Conditions: Freshwater, 74o to 79o F
  • Max. Size: 3 inches
  • Lifespan: 2 to 3 years

35. Scarlet Badis (Dario dario)

Scarlet Badis (Dario dario)
Learn more : flickr.com
  • Family: Osphronemidae
  • Care Level: Easy
  • Temperament: Semi-aggressive, species-only tank recommended
  • Diet: Primarily carnivorous
  • Water Conditions: Freshwater, 68o to 79o F
  • Max. Size: 0.8 inch
  • Lifespan: 3 to 6 years 

36. Dwarf Puffer (Carinotetraodon travancoricus)

Dwarf Puffer (Carinotetraodon travancoricus)
Image Source : wikimedia.org
  • Family: Tetraodontidae
  • Care Level: Intermediate
  • Temperament: Aggressive
  • Diet: Primarily carnivorous, prefers live foods
  • Water Conditions: Freshwater, 77o to 82o F
  • Max. Size: 1.4 inches
  • Lifespan:  4 years

37. Swordtail (Xiphophorus hellerii)

Swordtails (Xiphophorus helleri)
  • Family: Poeciliidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 65o to 82o F
  • Max. Size: 6 inches
  • Lifespan: 5 years

38. Spotted Blue-eye (Pseudomugil gertrudae)

Spotted Blue-eye (Pseudomugil gertrudae)
Image Source : wikimedia.org
  • Family: Pseudomugilidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73o to 86o F
  • Max. Size: 1 inch
  • Lifespan: 3 to 5 years

39. Dwarf Sucker (Otocinclus macrospilus)

Dwarf suckermouth catfish. You might think he looks like a tasty mouthful, but look closer and you'll see he's completely covered in spines.
Learn more: flickr.com
  • Family: Loricariidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 73o to 86o F
  • Max. Size: 1.8 inches
  • Lifespan: 3 to 5 years

40. Goldfish (Carassius auratus)

How Long do Goldfish Live? - Two Fantail goldfish with a blurry leaf background.
  • Family: Cyprinidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 68o to 74o F
  • Max. Size: 6 inches but depends on species
  • Lifespan: 10 to 20 years

41. Bristlenose Catfish (Ancistrus)

Bristlenose Pleco scraping tank surface.
  • Family: Loricariidae
  • Care Level: Easy
  • Temperament: Aggressive as adults
  • Diet: Omnivore, algae-eaters
  • Water Conditions: Freshwater, 73o to 80o F
  • Max. Size: 3.5 inches
  • Lifespan: 12 years

42. Common Molly (Poecilia sphenops)

Neon Orange Molly Fish
Image Source: wikimedia.org
  • Family: Poeciliidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 72o to 78o F
  • Max. Size: 4.5 inches
  • Lifespan: 5 years

43. Mystery Snail (Pomacea bridgesii)

Golden Mystery Snail - 1/2"-2" - Algae Eater for Aquariums
  • Family: Poeciliidae
  • Care Level: Easy
  • Temperament: Peaceful
  • Diet: Omnivore
  • Water Conditions: Freshwater, 72o to 78o F
  • Max. Size: 4.5 inches
  • Lifespan: 5 years

Conclusion

I hope you enjoyed this guide on the best fish and invertebrates to keep in a 20-gallon tank. A 20-gallon tank is the ideal size aquarium for a beginner, allowing you to enjoy caring for lots of different species without taking up too much space in your home. 

How did you stock your 20-gallon fish tank? We’d love to know!

Tell us about your aquarium in the comments box below, and if you found this article helpful, please share it!

Alison Page has been an avid fish keeper for over 35 years and has owned many different species of freshwater tropical fish including bettas. Currently Alison has two large freshwater tanks. The first tank has two huge fancy goldfish who are almost ten years old and still looking as good as ever. In the other, she has a happy community of tiger barbs, green tiger barbs, corydoras catfish, platys, and mollies.

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