How Many Fish in a 20-Gallon Tank?

Find out how many fish can you put in a 20-gallon tank! This comprehensive guide aims to help you determine how many fish your tank can accommodate and make the best choices for a thriving aquatic environment.

how many fish 20 gallon tank

We’ll cover various factors to consider, popular fish combinations, and tank setup tips to ensure your aquarium remains lively and beautiful.

What are the Dimensions of a 20-Gallon Tank?

A 20-gallon fish tank is a popular choice for many aquarium enthusiasts as it provides ample space for a variety of fish species and beautiful aquascaping. But before you decide on the number of fish you can accommodate in a 20-gallon tank, it’s crucial to understand its dimensions.

There are two common types of 20-gallon tanks: the 20-gallon long tank and the 20-gallon high tank. Each type has slightly different dimensions which impact the amount and types of fish that can comfortably live in the tank.

  • 20-Gallon Long Tank: The dimensions of a 20-gallon long tank are typically around 30 inches long, 12 inches wide, and 12 inches tall.
  • 20-Gallon High Tank: These dimensions are typically around 24 inches long, 12 inches wide, and 16 inches tall.

These dimensions might slightly vary across different brands and models. On the other hand, 20-gallon tanks made of acrylic material may be lighter and offer more shape options than standard glass aquariums.

It is essential to know that the length and width play a more significant role in determining the types of fish that your aquarium can accommodate.

Length and width determine the swimming area, which is vital to ensure your fish have enough space to exhibit their natural behavior. The height of the tank is more relevant for aquascaping and providing different levels for various fish species to inhabit.

Understanding the dimensions and layout of your specific 20-gallon tank helps to create a healthier and happier environment for your fish. So make sure to keep these factors in mind when making decisions about the number of fish and their specific needs.

What Factors Influence Fish Count in a 20-Gallon Tank?

When determining how many fish you can add to your 20-gallon tank, a few key factors come into play. These factors will ultimately influence the fish count and overall health of your aquarium.

Size of Fish: One of the most important factors to consider is the adult size of your fish species. Keep in mind that different fish will grow to various sizes, and some may become too large for a 20-gallon tank as they mature. Always research the adult size of the fish you plan to keep in your tank and take into account their growth potential.

Schooling Fish: Many fish species naturally live in groups or schools. If you add schooling fish to your tank, they’ll require more space than solitary fish so they can interact and swim together comfortably.

Fish Temperament: The temperament of your fish also plays a significant role in determining how many fish your tank can hold. Some fish are more aggressive and territorial than others, requiring more space to avoid conflicts.

Filtration Capacity: The effectiveness of your tank’s filtration system will also influence how many fish you can have in your 20-gallon aquarium. Ensure that your filter is properly sized and well-maintained to handle the bioload of your fish.

Aquarium Décor and Plants: The presence of live plants, rocks, and other décor in your aquarium will play a role in determining the number of fish you can safely house. These elements can provide necessary hiding spots and territories for your fish, contributing to a more comfortable and stress-free environment.

However, adding too many decorations can reduce the swimming space and lead to overcrowded conditions.

How Many Fish Can a 20-Gallon Tank Hold?

Determining how many fish a 20-gallon tank can hold depends on several factors. In general, the standard rule of thumb to follow is one inch of fish per one gallon of water.

  • Remember that fish grow over time, so consider their adult size when calculating.
  • Take into account the fish’s swimming habits, behaviors, territory requirements, and aggressiveness.

For most community fish that are compatible in terms of size and behavior, you can expect a 20-gallon tank to safely hold:

  • 10-12 small fish (1-3 inches in adult size)
  • 6-8 medium fish (3-5 inches in adult size)
  • 2-4 large fish (over 5 inches in adult size)

However, this is a general guideline and can vary depending on the specific fish species and their needs.

It is important to research the chosen fish species thoroughly before introducing them to your tank

Fish size Amount of fish in 20-gallons
Small 10-12
Medium 6-8
Large 2-4

To ensure a healthy and stress-free environment for your fish, always observe their behaviors and monitor water quality. Overcrowding a tank can lead to aggression, stress, and health problems. Be prepared to upgrade to a larger tank, or relocate some fish if needed.

Get familiar with your fish’s requirements and follow a proper stocking plan to avoid issues in the long run.

How to Choose the Right Fish for a 20-Gallon Tank?

When choosing fish for a 20-gallon tank, there are several factors to consider in order to ensure compatibility, proper space utilization, and a healthy environment. Size, temperament, and water type preferences should play a significant role in your decision-making process.

  • Size: The size of adult fish should be taken into account when choosing your fish species. Small to medium-sized fish work best in a 20-gallon tank, like tetras, guppies, and mollies.
  • Temperament: Choose fish that have similar temperaments to prevent violence or stress within the tank. Alternatively, you may consider fish that are known to be peaceful community dwellers. For example, neon tetras and harlequin rasboras could live harmoniously in a 20-gallon tank.
  • Water type: Take into consideration whether the fish you choose will reside in a freshwater or saltwater environment. Mixing freshwater fish species with saltwater fish species is not recommended. Stick with one type to provide proper water conditions for your tank inhabitants.
  • Schooling fish versus solitary fish: Many fish species prefer to live in groups, called “schools.” Choose a mix of schooling fish and solitary fish (if room permits) to create an engaging environment.

Here is a table with some popular fish choices for a 20-gallon tank:

Fish Species Size (Adult) Temperament Water Type
Guppies 1-2 inches Peaceful Freshwater
Mollies 3-4 inches Peaceful Freshwater
Neon Tetras 1-2 inches Peaceful Freshwater
Harlequin Rasboras 1.5-2 inches Peaceful Freshwater
Dwarf Gouramis 2-3 inches Peaceful Freshwater
Clownfish 3-4 inches Semi-aggressive Saltwater

Remember, it’s essential to choose the right combination of fish that will coexist peacefully and thrive within the limited space of a 20-gallon tank. Do your research carefully before making your final decision.

One of the most exciting aspects of setting up a 20-gallon fish tank is choosing the fish you’ll include in your aquatic environment.

When it comes to selecting the right fish combinations for your 20-gallon tank, you’ll want to consider fish species that have similar water parameters, are of similar size, and possess complementary temperaments. With these factors in mind, let’s explore some popular fish combinations for a 20-gallon tank.

Community Tank with Small, Peaceful Fish: If you prefer a community of small, colorful fish in a harmonious environment, consider stocking your 20-gallon tank with a combination of the following fish species:

  • Neon Tetras
  • Zebra Danios
  • Platies or Guppies
  • Corydoras Catfish
  • Dwarf Gouramis

This community will provide a visually appealing and active tank, while ensuring that all members live harmoniously together due to their peaceful nature.

Mixed South American Tank: For a different approach, you can set up a biotope tank with South American species. Recommended fish combinations for this type of tank include:

  • Dwarf Cichlids (e.g., Apistogramma or Ram Cichlids)
  • Small Tetras (e.g., Cardinal or Rummynose Tetras)
  • Otocinclus Catfish
  • Marbled Hatchetfish (for surface dwellers)

Be mindful of the number of fish you choose to include: too many fish will create a stressful environment and result in poor water quality. As a rule of thumb, ensure that the one inch of fish per gallon rule is followed, taking into account the adult size of each fish.

By understanding the preferences and requirements of each fish species, you’ll be able to create an attractive and diverse 20-gallon tank that supports healthy fish populations. Popular fish combinations, such as those listed above, are a great starting point but don’t be afraid to experiment and find the perfect mix for your personal preferences and tank setup.

Which Fish Species Are Not Suitable for a 20-Gallon Tank?

It is essential to understand that not all fish species are suitable for a 20-gallon tank. Some fish grow too large, while others need more space to thrive. Here is a list of fish species that are not recommended for a 20-gallon tank:

  1. Oscar fish: These beautiful fish can grow up to 12-14 inches and need at least a 75-gallon tank to thrive.
  2. Arowana: Known for their striking appearance, Arowanas can grow up to 3 feet and require a minimum tank size of 150-200 gallons.
  3. Plecostomus: Commonly found in many aquariums, these algae eaters can grow up to 18 inches and need a minimum tank size of 100-150 gallons.
  4. Goldfish: Surprisingly, goldfish are not suitable for a 20-gallon tank, as they can grow up to 12 inches, produce a large amount of waste and require a minimum of 40-50 gallons when grown.
  5. Cichlids: Many Cichlid species can grow quite large and are territorial, requiring a larger tank for suitable living conditions.
  6. Tinfoil barbs: These schooling fish can grow up to 12-14 inches and need at least 75 gallons for a small school.

When planning your 20-gallon tank setup, it’s important to consider the long-term growth and needs of the fish species you choose. Research the adult size, social behaviors, and minimum space requirements of each fish species before adding them to your tank.

Avoid overcrowding your tank with fish that will eventually outgrow the available space. This not only makes for unhealthy tank conditions, but it may also cause harm to the fish themselves.

What Live Plants to Use in a 20-Gallon Tank?

Adding live plants to your aquarium can be a great way to enhance its visual appeal and provide a natural habitat for your fish. Live plants can also help improve water quality by absorbing ammonia and nitrates, and they also provide hiding spots for your fish. In a 20-gallon tank, you have a variety of plant options to choose from.

Low-light plants are ideal for beginner aquarium hobbyists, as they require very little maintenance and typically thrive in most tank conditions. Some examples of low-light plants include:

  • Java moss
  • Java fern
  • Anubias
  • Cryptocoryne

For those with more experience, medium-light plants can be an excellent addition to their tank. These plants require slightly more care and attention, but their vibrant colors and interesting shapes can make them worth the extra effort. Popular medium-light plants are:

  • Amazon sword
  • Dwarf sagittaria
  • Vallisneria
  • Rotala

If you’re up for a challenge, high-light plants can bring your aquarium to life with their vibrant colors and lush foliage. These plants require a strong light source and may require additional care, such as supplementation with CO2 or fertilizers. High-light plants include:

  • Dwarf baby tears
  • Hairgrass
  • Madagascar lace plant
  • Alternanthera

Determining the right live plants for your 20-gallon tank largely depends on your experience level, the type of fish you plan to keep, and your tank’s lighting conditions. With the right mix, you can create a stunning underwater environment for both you and your fish to enjoy.

How to Set Up and Maintain a 20-Gallon Fish Tank?

Setting up and maintaining a 20-gallon fish tank can be a fun and rewarding experience. Follow these simple steps to create a healthy and vibrant environment for your fish.

  1. Choose a location: Find a safe and stable spot in your home for your tank, away from direct sunlight and drafts.
  2. Install essential equipment: This includes a heater, filter, thermometer, and lighting.
  3. Add substrate: Spread a layer of gravel or sand on the bottom of the tank.
  4. Arrange decorations: Decorate your tank with rocks, driftwood, and live or artificial plants to provide hiding places and visual interest.
  5. Fill the tank: Fill your tank with dechlorinated water, leaving some space at the top for air exchange.
  6. Add beneficial bacteria: These help maintain water quality and reduce ammonia levels. You can add them by using a bottled bacteria starter.
  7. Cycle the tank: Allow the tank to run for at least 2 weeks before adding fish, helping the beneficial bacteria establish themselves.

Regular maintenance is crucial for a healthy tank. Perform these tasks on a routine basis:

  • Water changes: Replace 25-30% of the water in the tank every 2-4 weeks to maintain water quality.
  • Water tests: Check pH, ammonia, nitrite, and nitrate levels regularly, adjusting as needed.
  • Clean the filter: Rinse your filter media in tank water to remove debris and preserve beneficial bacteria.
  • Remove algae: Clean algae from the tank walls and decorations using an algae scraper or pad.
  • Siphon the gravel: Use a gravel vacuum to remove debris and waste from the substrate.

By following these guidelines, you can ensure that your 20-gallon tank remains a thriving environment for your fish.

How to Maximize Space in a 20-Gallon Tank?

One of the most essential things to consider when setting up a 20-gallon tank is making the best use of available space. It is crucial not only for aesthetic purposes but also for ensuring that your fish have the optimal living conditions. Here are some tips on how to maximize space in your 20-gallon tank:

  • Choose the right decorations: When selecting decorations for your tank, choose ones that are functional and space-saving. This could include rocks, caves, or driftwood that can serve as hiding places for fish, or even serve as surfaces for plant growth.
  • Add vertical elements: Including taller decorations or live plants can help draw the eye upward and create the impression of a larger tank. This also provides fish with more space to swim and explore different levels.
  • Arrange decorations wisely: Strategically arranging your decorations can help utilize the tank’s space effectively. Position larger items towards the back or sides of the tank to avoid overcrowding the central area.
  • Select proper live plants: When adding live plants, choose species with different growth patterns to create a visually appealing and biologically diverse environment. Some plants grow tall, such as Vallisneria or Amazon sword, while others fill in horizontally, like Anubias and Java moss.

By following these tips, you’ll ensure your 20-gallon fish tank appears more spacious and provides a comfortable environment for your aquatic friends. Maximizing your tank’s space also helps reduce stress among fish and fosters a healthier, more thriving ecosystem.

FAQs about 20-Gallon Tanks

What is the best filter for a 20-gallon tank?

A high-quality filter is essential for maintaining a healthy aquarium environment. For a 20-gallon tank, consider a hang-on-back filter or a canister filter rated for the tank size.

What heater should I use for a 20-gallon tank?

It’s crucial to choose a heater with the appropriate wattage for your tank size. For a 20-gallon tank, a 50-100 watt heater is recommended to maintain stable water temperatures.

How often should I change my 20-gallon tank’s water?

Partial water changes are essential to maintain water quality. For a 20-gallon tank, aim for a 20-30% water change every two to four weeks, depending on fish load and water conditions.

What are some popular fish species suitable for a 20-gallon tank?

Some popular fish for a 20-gallon tank include small community fish such as guppies, platies, tetras, and mollies. Some other options include shrimp, snails, and dwarf species like dwarf gouramis and pygmy corydoras.

Can I keep live plants in my 20-gallon tank?

Yes, live plants not only improve the aesthetic appeal of your tank but also provide natural filtration and oxygenation. Popular choices for a 20-gallon tank include java fern, anubias, cryptocoryne, and java moss.

Is a 20-gallon tank suitable for a beginner aquarist?

Yes, a 20-gallon tank is an excellent choice for beginners. It provides enough space to create a stable environment with relatively low maintenance, while also offering flexibility for various fish and plant species.


Choosing the right fish and arranging your 20-gallon tank effectively can create a thriving aquatic environment. Always consider the various factors that influence fish count and opt for compatible species that can maintain harmony in your tank.

We hope this comprehensive guide has been helpful – feel free to leave a comment to share your thoughts or experiences with 20-gallon tanks!

Fishkeeping   Updated: July 6, 2023
avatar Hello, my name is Fabian, and I am the Chief Editor at Aquarium Nexus. I have over 20 years of experience in keeping and breeding fish. The aquarium hobby brings me immense joy, and I take great pleasure in sharing my experiences with others.

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