How Many Fish in a 20 Gallon Tank

Wondering, how many fish can fit in a 20-gallon fish tank? The tank size is small so we can not put large fish in it.

We can still put small fish in this tank. This How Many fish in a 20 Gallon Tank guide will help you to understand what fish you can keep and which are not.

How Many Fish in a 20 Gallon Tank?

This is a small tank so we should keep small fish in this tank. Sometimes small tank is also called a nano tank.

There is a guiding strategy we can follow, 1-inch of fish per gallon of water. Though we can not use this guideline when stocking 3-4 inches of fish.

When stocking keeps in mind the fish should not be any bigger than 1 inch when they are adults.

Stocking Ideas for 20-Gallon Fish Tank

Here are some ideas for you to stock your tank:

Livebearer Community tank

  • 3X Guppies
  • 2X Platies
  • 2X Mollies

Tetra Community Tank

  • 5X Neon Tetras.
  • 5X Glowlight tetras.
  • 3X Black Phantom tetras.
  • 3X Cherry Shrimp.

Combo No.3

  • 6-10 Danios.
  • 6 Pygmy Corydoras.
  • 6 Cherry Shrimp.

Combo No.4

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

Combo No 5.

  • 1 pair of (male and female) German Blue Ram
  • 10-15 Ember Tetra
  • 4-6 Albino Corydoras Catfish

Combo No.6

  • 1 pair of Dwarf Gourami
  • 10-15 Glowlight rasbora
  • 8-10¬†Cherry Shrimp

Combo No.7

  • 4 female and 2 male Platies
  • 8-12 Rasboras or Tetras
  • 6-8 Dwarf Cory Catfish

Combo No.8

  • 1 pair of Dwarf Pufferfish
  • 8-12 Rasboras
  • 10-15 Ramshorn Snails

Combo No.9

  • 4 female and 2 male Guppies
  • 8-12 Rasboras
  • 6-8 Cory Catfish

Combo No.10

  • 10 Danio Margaritatus
  • 6-8 Corydoras

Best Fish to Keep In a 20-Gallon Tank

To stock our nano tank, we will need to choose peaceful community fish. Tank size does limit our options but there is plenty of beautiful small fish to choose from.


Tetras are one of the most popular small fish. They are shoaling fish. They do their best when they are in groups. Keep at least 6 of them. But if you want a school then keep at least 15 or more together.

Care LevelEasy
Size1.5 inches
How many tetras can you keep in a 20-gallon tank?Up to 16


Guppies are eyes pleasing. Their variant color can light up your aquarium. If you want their full color, pair them with a 1:2 male-to-female ratio. Sometimes males are harassed by females so you can put 2-3 females for one male. This will help to reduce stress levels.

Care LevelEasy
Diet Omnivorous
How many guppies can you keep in a 20-gallon tank?10


Betta splendensa

Betta is semi-aggressive fish. You should not put them in a community tank with small fish. If you want to keep them, you should only keep betta fish.

Care LevelEasy
Size2- 3 inches
How many bettas can you keep in a 20-gallon tank?1 male or 2 females


Cherry Barb
Cherry Barb

Barb has red coloring covering their whole body. They are peaceful schooling fish. Males can get aggressive when wanting to breed. Keep a ratio of one male to two females. The bigger the school, the more active and playful they will be.

Care LevelEasy
Size2.8-3.9 inches
How many barbs can you keep in a 20-gallon tank?6


Image source:
Care LevelEasy
SizeUp to 4.5 inches
Life span5 Years
How many mollies can you keep in a 20-gallon tank?4-6


Celestial Pearl Danio
image source:
Care LevelIntermediate
SizeUp to 1 inch
Life span 3-5 Years
How many rasboras can you keep in a 20-gallon tank?16


Image source:
Care LevelEasy
Size2-3 inches
Life span3-4 years
How many platy can you keep in a 20-gallon tank?10

Pygmy Cory

How Many Fish in a 20 Gallon Tank
Image source:
Care LevelEasy
Size2.5 inches
Life SpanUp to 10 years
How many Pygmy Cory can you keep in a 20-gallon tank?6-8

Factors that you should keep in mind before adding any fish to a 20-gallon tank

The “1 inch of fish per gallon of water” guideline is not that accurate. Especially when it comes to stocking bigger fish (4 inches or bigger), but When it comes to stocking small fish around 1 inch, it is accurate.

Tank setup can also affect how much fish you can keep in a 20-gallon tank:

  • Water Quality
  • Oxygenation
  • Tank Dimensions

If you want to make a community fish tank. Keep these things in mind when stocking your fish.

  • Compatibility between different fish species.
  • Adult fish size (no more than 2 inches)
  • What part do they tend to swim in?
  • Territoriality or aggression levels.
  • Shyness.
  • Shoaling behavior.

Always keep in mind that every fish you choose should have preferences in water conditions. You should not keep cold water fish with tropical fish.

Keep in mind you should not overcrowd your aquarium. Overcrowing fish may lead to stress and poor water conditions. This will affect your fish’s health.

Best Live Plants For a 20-Gallon Tank

We have less room to work with when it comes to planting live plants in a 20-gallon aquarium. But planted tanks are beautiful. Live plants also improve water quality and oxygenate the tank.

Live plants also provide hiding places for fish. This will help them to feel less stressed. Due to a lack of space, we can not plant many plants. Here is a list of popular live plants for small tanks:


When you use 1 inch of fish per gallon of water, you should keep in mind the size. The adult fish size should be 1 inch.

In-store you can find 1-inch or small juvenile fish. you should not add them using the “1 inch of fish per gallon of water”.

Hey, I am Shuvradeb Biswas a content writer. Fishkeeping is my hobby. There are many problems I faced during my first fishkeeping. So, I made the blog to help new fishkeepers.

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