How Many Fish Per Gallon? (Answers for Different Tank Sizes)

A group of fish in a 25 gallon fish tank

One of the most important things in setting up a fish tank is how many fish per gallon it can sustain. After all, it’s more than just filling the aquarium with water and fish; it’s also about creating a balanced environment.

In this guide, we’ll look at common tank sizes and determine the optimal number of fish per gallon for each. We aim to provide deeper insights, moving past the basic one-inch-per-gallon rule.

How Many Fish Per Gallon?

Orange and gold fish in a sunlit tank

When deciding how many fish to put in an aquarium, you should consider the size and activity level of the fish, the amount of waste they create, and their compatibility with others. Though it’s common to hear about the one-inch-per-gallon rule, this misses the important factors mentioned.

Likewise, the shape of your aquarium is an important consideration. In general, it must allow your fish to move freely and comfortably, taking into account their size and natural movement in the wild.

For instance, while zebra danios are small, their active nature means they need a larger tank to swim around.

The amount of dissolved oxygen and the bioload each fish contributes also dictate the size of the tank they need. Larger or more active fish require more space, as overcrowding leads to a lack of oxygen and a buildup of toxins. 

It’s also important to understand the needs of different types of fish, such as schooling fish like neon tetras, which require a group environment and, therefore, a larger tank despite being relatively small. 

Integrating live plants can help manage bioload by breaking down nitrates and improving water quality. However, it also takes up a significant amount of aquarium space, leaving less room for your fish. 

Simply put, there’s no single answer to how many fish per gallon you should have. It’s a balancing act that requires fish keepers to consider many factors carefully. 

Problems With the One-Inch-Per-Gallon Rule

Goldfish swimming near decoration in a tank

The one-inch-per-gallon rule for fish stocking is now seen as outdated and too simple. It doesn’t fully consider each fish species’ needs, such as size, activity level, and how they get along with other species. 

Instead of relying on the one-inch-per-gallon rule, it’s better to consider a handful of factors when stocking an aquarium. These factors include species-specific needs, tank size and shape, and bioload and filtration. 

Whenever I set up new tanks, I find tools such as AqAdvisor to be a useful basis. These calculators provide a more accurate idea of how many fish a tank can hold. 

It goes beyond the basic one-inch-per-gallon rule, considering factors like fish size, behavior, species compatibility, and your tank’s layout and filtration capacity. However, it is still far from perfect.

Pro Tip: If you’re just starting with fishkeeping, the one-inch-per-gallon rule can be a passable basis. But remember, as your tank inhabitants grow and your tank changes, you might need to adjust the tank size. 

General Guidelines for Different Tank Sizes

A school of dark blue fish in a 15 gallon fish tank

How Many Fish Can I Have in a 2 Gallon Tank?

In a 2-gallon tank, it is generally safe to keep 1 to 2 small fish, each growing up to about an inch in length. 

This tank size is best for very small species or juvenile fish, as the limited space and water volume can’t support a larger bioload.

How Many Fish Can I Have in a 5 Gallon Tank?

For a 5-gallon tank, a good rule of thumb is to keep no more than 2 to 5 small fish, each about an inch in size when fully grown. 

This tank size allows for a little more flexibility but still requires careful management to maintain water quality.

How Many Fish Can I Have in a 10 Gallon Tank?

In general, a 10-gallon tank can comfortably accommodate 5 to 10 small fish, each about one inch long, or 2 to 3 medium-sized fish, each around 1 to 3 inches when fully grown. 

This tank size provides more options for setting up a diverse aquatic environment.

How Many Fish Can I Have in a 15 Gallon Tank?

In a 15-gallon tank, you can house about 7 to 15 small fish, each roughly an inch in length, or 3 to 5 medium-sized fish, each about 3 inches long when fully matured. 

This gives you the flexibility to mix different species without overcrowding.

How Many Fish Can I Have in a 20 Gallon Tank?

A 20-gallon tank can support 10 to 20 small fish, each about one inch in size, or 4 to 6 medium-sized fish, each roughly three inches long when full-grown. 

This size is great for a small community aquarium with a variety of species.

How Many Fish Can I Have in a 30 Gallon Tank?

A 30-gallon tank offers more space and can comfortably hold up to 30 small fish, each about an inch in size when fully grown. 

Alternatively, it can accommodate up to 10 medium-sized fish, each up to three inches long at maturity. This tank size provides ample room for a diverse mix of fish and more elaborate aquatic setups.

How Many Fish Can I Have in a 50 Gallon Tank?

In a 50-gallon tank, you have the capacity for 25 to 50 small fish, each approximately an inch in size, or 10 to 15 medium-sized fish, each ranging from 3 to 4 inches in length at full size. 

This size is ideal for creating a vibrant and varied community of fish, as it offers plenty of space for different species to coexist.

How Many Fish Can I Have in a 55 Gallon Tank?

A 55-gallon tank can hold about 27 to 55 small fish, each roughly an inch in size at full growth, or 11 to 18 medium-sized fish, each between 3 and 4 inches when fully matured. 

For larger fish, which can range from 6 to 12 inches, this tank size can house around 4 larger fish, depending on the species and their specific needs.

This tank size is excellent for housing a diverse array of fish species, providing enough room for both active swimmers and more tranquil species.

How Many Fish Can I Have in a 75 Gallon Tank?

With a 75-gallon tank, you can stock up to 35 to 75 small fish, each around about an inch in size when fully grown, or 15 to 25 medium to large fish, each 4 to 6 inches at maturity. 

 For larger species, this tank can hold about 5 to 6 large fish, assuming they grow to be about 8 to 12 inches. This size is perfect for keeping larger or more active fish, offering ample space for swimming and interaction.

Watch this video to learn more about how many fish you can add to an aquarium: 

How MANY FISH Can You Put In Your Aquarium?

Fun Fact: Aquarium stocking refers to the process of determining the appropriate number and types of fish to keep in an aquarium. 

It involves careful consideration of factors like tank size, fish species, and the environmental needs of the fish.

How to Choose Fish for Your Aquarium Tank

Various fish with greenery filled tank

If you already have a specific tank size in mind and are looking to select different fish for it, here are some simple steps to guide you through the process:

Consider the size and layout of your tank

The dimensions of your aquarium greatly influence the type of fish you can keep. Larger, wider tanks can accommodate more or bigger fish, while taller, narrower tanks are more restrictive.

If you have a narrow tank, it might be best to keep smaller fish species that do not need a lot of room to move around. 

Remember, the space taken up by plants and decorations also affects the available room for your fish, so take this into consideration.

Research fish size and growth 

Look up the adult size of the fish you’re interested in. Use the one-inch-per-gallon rule to get a clue if your chosen fish will thrive in your aquarium or not. 

This should also give you an idea about how many of them will fit in your aquarium. 

Consider fish behavior and compatibility

If you’re planning to add more than one species of fish to your tank, pick species that can live together harmoniously. Remember, some fish might be territorial or aggressive, which could disrupt a peaceful tank environment.

It is recommended to keep fish with similar temperament and size together. For example, community tanks should have non-aggressive species that thrive in similar water conditions. 

Look up each species’ specific needs, including diet, water temperature, and pH levels, to ensure they are compatible. Avoid mixing predatory fish with smaller, peaceful ones, as this can lead to stress or harm.

Understand bioload and filtration needs

When picking fish for your aquarium, it’s important to think about how much waste they produce and if your tank’s filter can handle it. Different fish make different amounts of waste. Bigger or more active fish usually make more. 

So, choose fish that your filter can keep up with. A good way to get an estimate of a fish’s bioload is by using calculators such as AqAdvisor

Finally, despite having a good filter, you will still need to manually clean your tank from time to time. You can also use this as a deciding factor. If you want less maintenance time, pick fish with lower bioloads. 

Hopefully, this guide has helped you better understand the complex topic of aquarium stocking and the number of fish suitable for different tank sizes. If you have any tips regarding this, please feel free to leave a comment below!

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