How Many Fish Per Gallon (5, 10, 20 Gallon Included)

Aquarium setup is exciting for aquarists regardless of their experience, but it’s important to know how many fish can be kept per gallon in a tank to ensure success. And to do this properly, you need to be equipped with the right knowledge.

In this article, we’ll explore the factors that influence this crucial decision, helping you create a harmonious and healthy aquatic environment.

The One Inch Per Gallon Rule (Generally)

The “One Inch Per Gallon” rule is a well-known guideline frequently used by beginner aquarium enthusiasts to determine the number of fish that can be kept in an aquarium. It’s a simple and easy-to-understand concept, but like many rules in the world of fishkeeping, it comes with its own set of nuances and considerations.

The “One Inch Per Gallon” rule suggests that you can keep one inch of fish for every gallon of water in your aquarium. This rule is often used as a starting point for beginners when deciding how many fish they can safely house in their tank. For example, if you have a 20-gallon aquarium, you could theoretically keep 20 inches of fish.

Why The One Inch Per Gallon Rule Isn’t Always Accurate

The “One Inch Per Gallon” rule is a popular way for beginners to decide how many fish they can put in their aquarium. However, it’s a simple rule with some big limitations. 

Although the “One Inch Per Gallon” rule is a good starting point for stocking an aquarium, it’s not the only thing you need to consider. You also need to think about the specific needs of the fish species you want to keep, such as their adult size, behavior, and compatibility with other tankmates.

It’s also important to maintain excellent water quality to keep your fish healthy. Ultimately, successful fishkeeping is about careful planning and informed decisions, not just following a simple rule.

What Can Affect The One Inch Per Gallon Rule?

The “One Inch Per Gallon” rule, while straightforward in concept, can be influenced and affected by several factors that determine whether it’s suitable for your specific aquarium setup. Let’s break down how each of these factors can impact the rule:


A powerful and efficient filter can help manage the waste produced by your fish. A robust filtration system can potentially allow you to house more fish in your aquarium, even if you’re close to the one-inch-per-gallon limit.


Live plants can also act in a similar way to the filter, removing harmful chemicals like ammonia from the tank, as well as oxygenating it. They can also provide additional hiding spots and territories for fish, potentially allowing for a higher fish density.

Fish Size

The adult size of the fish you intend to keep is an important factor. Larger fish naturally require more swimming space and generate more waste. Consider the eventual size of your fish when applying the rule.

Tank Size

The size of your aquarium also matters. A larger tank provides more swimming space and dilutes waste more effectively, allowing for a greater number of fish. Smaller tanks may require stricter adherence to the rule.

Experience Level

The more you know about fishkeeping, the better you can take care of your fish. Experienced fishkeepers know how to keep their aquariums healthy and can experiment with different setups and care routines.

Tank Setup

The way you set up your fish tank, including the decorations, hiding places, and layout, can affect how well your fish get along. A well-designed tank can help keep your fish happy and healthy.

Aggression Levels

Some fish are more aggressive than others. Aggressive fish may require more space to establish territories and reduce stress on tankmates. Understanding the personalities of your fish is essential when applying the rule.

Swimming Space

Fish that are strong swimmers or have high activity levels may need more swimming space than others. Consider the behavior of your fish type and how they utilize the space in your tank.

The Shape Of Your Tank

The shape of your aquarium can affect fish behavior and swimming patterns. Long, rectangular tanks often provide better swimming space than tall, narrow ones.

How Much Waste The Fish Produce

The amount of waste produced by fish varies. Fish that produce a lot of waste may need stricter water quality management practices, which could affect the application of the rule.

FactorImpact on Rule
FilterA robust filter can allow for more fish.
PlantsLive plants can improve water quality and allow for higher fish density.
Fish SizeLarger fish require more space and produce more waste.
Tank SizeLarger tanks can accommodate more fish.
Experience LevelMore knowledge allows for better care and potentially more fish.
Aggression LevelsAggressive fish may require more space to reduce stress on tankmates.

How Many Fish In A 5 Gallon Tank?

A 5-gallon tank is relatively small, so it’s essential to be mindful of the fish type and their specific requirements when determining how many fish can comfortably inhabit it.

However, a 5-gallon tank is a great size for small fish, like ember tetras, betta fish, chili rasboras, and shrimp. I’d recommend keeping 2-4 fish of one inch or less in length in a 5-gallon tank.

Fish TypeMax Size (inches)Lifespan (years)Ideal Number in 5-Gallon Tank
Chili Rasbora1.254-83-4
Dwarf Pea Puffer1.44-51 (Species only tank)
Least Killifish1.233
Strawberry Rasbora0.783-4
Dwarf Rasbora2.53-53
Asian Stone Catfish1.551-2
Clown Killifish1.42-33
Betta Fish2.5-32-41

How Many Fish In A 10 Gallon Tank?

In a 10-gallon tank, you have a bit more room to work with compared to a 5-gallon tank, but it’s still a relatively small aquarium. The number of fish you can keep in a 10-gallon tank depends on several factors, including the species of fish, their adult size, and their compatibility.

Generally, I’d recommend keeping between 5-6 fish in a 10 gallon tank, such as guppies, harlequin rasboras, or celestial pearl danios. This gives your fish enough space to swim happily, and you can make mistakes that won’t be fatal.

Some fish are more active than others and require more space to swim. For example, you would not want to keep a school of neon tetras in a 10-gallon tank, as they would quickly outgrow the space and become stressed.

The adult size of the fish is also important to consider. Some fish, such as goldfish, can grow quite large and would need a much larger tank than 10 gallons.

Finally, you need to make sure that the fish you choose are compatible with each other. Some fish are aggressive and will attack other fish, so it’s important to do your research before adding any fish to your tank.

If you’re unsure about how many fish you can keep in your 10-gallon tank, it’s always best to err on the side of caution and keep fewer fish. This will help to ensure that your fish are healthy and happy.

Fish SpeciesMax Size (inches)Lifespan (years)Number in 10-Gallon
Betta Fish2.5-33-51
Cherry Barbs1-24-65-6
Zebra Danios1.5-23-55-6
Dwarf Gourami2-3.54-61-2
Corydoras Catfish1-2.55-74-6
African Dwarf Frog1.552-3

How Many Fish In A 20 Gallon Tank?

When you get to 20 gallons, you can begin to keep even MORE fish in your tank! Generally I’d say you should aim to keep around 15 small fish in a 20 gallon tank. At this point, though, you really are spoilt for choice with what you can keep.

Most tetras, rasboras, killifish, danios and more can live in a 20 gallon tank, just make sure you’re checking their compatibility and have fun!


Is A 5 Gallon Tank Okay For 1 Goldfish?

A 5-gallon tank is too small for most fish, especially goldfish. Goldfish produce a lot of waste and can grow large. A 20-gallon tank is the minimum size for a single goldfish, and larger is better.

How Many Guppies Can You Put In A 5 Gallon Tank?

A 5-gallon tank can hold 2 female guppies and 1 male guppy. More than this will overstock the tank and lower water quality. Guppies breed quickly, so a 10-gallon tank is still recommended.

How Many Tetras Can Go In A 5 Gallon Tank?

Most tetras, like neon tetras or cardinal tetras, are also not suitable for a 5-gallon tank. They require more space to swim and should be kept in larger tanks, preferably 10-15 gallons or more.

How Many Danios Can You Put In A 5 Gallon Tank?

Like tetras, danios are active swimmers and need more space. A 5-gallon tank is too small for most danio species. So, it’s recommended to keep them in tanks that are 10 gallons or larger.


When keeping an aquarium, prioritize the well-being of your fish over quantity. Each fish has unique requirements, and overcrowding can lead to stress, health problems, and poor water quality. Research your fish’s needs, ensure proper filtration and water parameters, and provide plenty of space to create a stunning and thriving underwater world.

About the author

Hey! I'm Antonio!

Betta fish keeper for over 6 years now! Since owning a betta I've also housed all kinds of tropical fish, and have seen all manner of problems and how to look after them!

If you need any advice you can always message me or better yet join the Facebook group where a community can answer your questions!

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