How Many Fish Can I Put In A 3-Gallon Tank?

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Taking care of fishes is one of the most rewarding experiences as a pet owner. There’s just something magical and exhilarating when choosing what size of tank and fish species to buy. 

It might be best for beginners to go for smaller fish tanks to help you acclimate and adapt to the hobby. A 3-gallon tank is the smallest variant available. Despite its small size, it can hold a variety of fish species. 

If you are wondering how many fish can I put in a 3-gallon tank, then you have come to the right place. 

small fish


How does a fish owner know how many fishes they can keep? 

Overstocking your aquariums results in disastrous outcomes since the fishes will only be stressed and might even acquire diseases due to the cramped size. To avoid such, here are some factors you need to consider. 

One inch per gallon 

This is the most famous rule when it comes to stocking your fish tank. It states that you need to allocate at least one gallon for every one inch of fish. As such, a 3-gallon tank can more or less hold three fish species.

However, do remember that this calculation only gives a rough estimation. You will have to consider the amount of space your aquarium’s equipment, plants, and other decorations will occupy to get a more accurate measurement. 

Indeed, the more plants, wood, rocks, gravel, and similar decorations your fish tank holds, the lesser space it will have. For a more accurate measurement, you have to determine those measurements and deduct them from the size of the fish tank. Usually, the fish tank often holds 10% to 15% less water volume than its actual size. 

You will also have to consider the difference in your fishes’ size. Keeping ten inches of slender-bodied fishes is not the same as stocking your tank with ten inches of broad and full-bodied species. As a rule, larger fishes occupy more space and need more water volume to move around. 

Moreover, take note that fishes will also grow. The volume your baby fishes occupy will gradually increase as it grows into an adult. Many fish tank owners, however, have no clue as to how large their fishes can grow. 

Before buying any fish, make sure that you have done your research. Some pet shops also provide signages and information boards regarding the fish’s adult size, compatibility with other species, and the like. Knowing this information is crucial for you to make the necessary adjustments. 

Surface Area 

Fish tanks with larger surfaces facilitate more oxygen exchanges, which in turn supports more fishes. In other words, a larger surface area would increase the number of fish you can keep in your tank. 

Compared to the one inch per gallon rule, the surface area rule considers the difference between fish tank shapes. You can get your aquarium’s surface area by multiplying the width and length of your tank. This rule dictates that you can keep one inch of fish for every 12 square inches of your aquarium. 

However, the surface area also has its flaws. It mainly operates on the assumption that you are keeping slender-bodied fishes, which is not always true. If you have larger fishes, you can adjust the ratio to one inch of fish for every 20 square inches of your fish tank. 

One inch per gallon rule vs. surface area rule: which is better? 

The one inch per gallon rule applies to most situations, and it’s easy to calculate and use. However, if you are using a non-standard sized fish tank, you are better off using the surface area rule. 

Regardless of what rule you abide by, always make sure to do your research. It’s safer to take as little space as possible rather than going over. 

Moreover, make sure that you do not fully stock your aquarium in one go. No more than 25% of fish species should be introduced. With this method, you can enable beneficial bacteria to grow and steadily acclimate the fishes to the new environment. 

Filtration Systems 

Filtration systems also affect the number of fishes you can keep inside your tank. The filter should run four times the total water volume inside your tank per hour to support your aquarium. 

When in doubt, it’s best to go with more frequencies as there is no danger of over-filtering your tank’s water. 

aquarium (2)


What are some fish types I can add to my 3-gallon fish tank? 

Smaller fish tanks are recommended for beginners since they are easier to take care of and maintain. It helps new owners adjust and familiarize themselves with the notions. 

However, considering its small size, there’s also a limited number and type of fishes you can keep inside. Here are some of the popular ones: 

Betta Splendens 

Bettas are known for their colorful bodies and long trailing fins, making them one of the most attractive fish types for aquariums. However, make sure you only keep one betta fish in your tank since they are not a friendly species. 

Betta fishes only grow up to 2 inches in length and are relatively easy to take care of. Being a member of the Anabandtodei or labyrinth fish group, they can breathe air using their mouths, allowing them to adjust in oxygen-poor conditions. 

Least Killifish 

The least killifish are olive-colored fishes, growing only up to 1.5 inches in length. They reproduce by laying eggs and are also easy to care for. 

They are known to be a shy species; hence, they might need some plants, rocks, and other decorative pieces to hide under. 

Fancy Guppy 

Guppies have distinct flowy tails and come in different colors. Having a school of at most 6 of these guys can make for an attractive and eye-catching aquarium. They are also easy to care for and are very social creatures. 

For best results, it’s best to have two female guppies for every male to help them relax. 

Endler’s Livebearer 

One of the more colorful types, these fishes come in striking stripes of colors in their bodies. They are incredibly fertile and reproduce by giving birth to live young. 

If you don’t want to have thousands of these guys, you have to separate their genders. Most fish tank owners keep 2 to 3 endler’s livebearers of the same gender in their tank. 

You can tell their sexes by their measurements. Males only grow up to 1 inch, while females can grow up to 1.5 inches. 

Neon Tetras 

Neon tetras are also tiny and colorful fishes. Like the guppies, they are an amiable type and can thrive well in groups of 5. 

However, they can be a bit sensitive. As such, you’ll have to keep the tank’s water clean and clear. 

Asian Stone Catfish 

While not as bright as other fish types, catfishes are known for their long, prominent whiskers. These species are native in Asia, especially in Bangladesh and India. 

They can grow up to 1 inch and are nocturnal animals. Catfishes have passive personalities so that it won’t hurt to keep at most 3 of them in your tank. 

Zebra Danio 

Zebra Danios are known for having stripes reminiscent of that of a zebra. They are tiny and sociable fishes, thriving mostly in groups of 5 or 6. They are also easy to care for due to their hardier nature. 

Pygmy Gourami 

Pygmy gouramis are the smallest in the gourami family, growing only to an average length of 1.5 inches. They have brightly colored bodies with two colored stripes. 

These fish types are docile and are relatively easy to care for. However, make sure that your tank is not affected by too much noise as this can agitate them. Avoid also using harsh lights as this can wash out their colors. 

Pea Puffer Fish 

Pea puffer fishes have prominent black spots on their bodies and are tiny, growing only up to an inch. These fish types, however, are territorial and aggressive. As such, you shouldn’t mix them with other species. 

Pea puffer fishes are also adventurous. They thrive well if there are various plants, rocks, and other decorative pieces they can explore. These fishes are also known to jump out of their tanks occasionally, so it’s best if you keep a lid on them. 

Bumblebee Goby 

Bumblebee goby fishes come in vibrant yellow and black stripes. They are tiny and thrive well in brackish waters. 

They are also energetic and active with a big appetite to match. This big appetite might not bode them well if they are kept with other species. 

While peaceful, they can be aggressive towards their kind. Hence, it’s best only to keep one. 

Scarlet Gem 

As in their name, scarlet gem fishes come in a vibrant red color accompanied by gold shades. They are shy around other fishes and are sometimes aggressive towards their kind. Hence, it’s best you only keep one of them. 

They thrive well if the aquarium comes with different decorations that enable them to hide. 


A 3-gallon fish tank, while easier to maintain, only offers a limited amount and species of fishes. Hopefully, this article has answered your query on how many fish can I put in a 3-gallon tank. 




Rita Wagener
Rita is a resident paw expert at Pet Keepers. A registered & licensed dog trainer, she also has a degree in animal nutrition, and runs her own dog training course.

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