How Many Fish Can You Keep In A 20 Gallon Tank?

A 20-gallon tank tends to be a popular choice for aquarium enthusiasts. Depending on the types of fish you’re hoping to keep, you can fit anywhere from a couple to multiple fish in a 20-gallon tank. A rule often followed is to stick with one inch of fish per gallon in a tank, but the reality is more complex than that.  

There are some fish that will do well in a tank this size, but how you set up your tank and what kinds of fish can or cannot thrive in a tank this size are just a few factors you need to consider when you purchase a 20-gallon tank. 

Is A 20 Gallon Tank Big Enough? 

A 20-gallon tank refers to the capacity of water that can fit inside the tank. This is a fairly standard size for small fish tanks suitable for a home. Whether or not this is enough space to set up your desired tank will vary depending on the fish you desire to have at home. If you’re relatively new to keeping fish, it’s best to start this small.

First, choose the kind of fish you want in your tank, and work around that fish to determine who else and what else can fit in the tank. You need to be able to accommodate your fish, create the ideal habitat for them, and leave them with ample space to swim around in. Overcrowding is a serious concern in an aquarium that can shorten the lifespan of your fish. 

There are certainly many options for fish tanks that exceed a 20-gallon capacity. Even fish that appear like they’re small enough to live in a tank this size won’t be happy, as they’ll likely want more room for swimming. There are many types of fish suited for a tank at this capacity, so you’re not limited in terms of creating a beautiful aquarium. 

Factors That Influence How Many Fish You Can Keep In A 20 Gallon Tank

There is no clear-cut number of fish that will always fit in a 20-gallon tank. You have to consider multiple factors before coming to a conclusion as to how many fish will safely and happily live in your tank. 

What Size Fish You Are Considering 

This will be one of the most important facts that will influence how many fish can be kept in a tank that has a capacity of 20 gallons. You’ll have to stick with choosing very small fish to keep in a tank with this capacity. 

How Big The Fish Become In Adulthood 

The fish you like might appear to be small now, but they might not stay that way forever. It’s good to know the average maximum size a fish can become once they reach adulthood before making your decision on how many to put in your tank. 

What Kind(s) Of Fish You’re Considering 

Not all fish are well-suited to a 20-gallon tank. Furthermore, even if the fish you want will adapt well to a tank this size, you also need to account for whether or not they need multiple fish in their aquarium to be happy, or they don’t do well having certain types of fish housed with them. 

The Dimensions Of The Tank

Since 20 gallons refers to the amount of water a tank can hold, a 20-gallon tank will come in a few different sizes. Some tanks with this capacity are long and short, others are square, and some are tall and not as long. 

The Decorations In The Tank 

The accessories, decorations, and other additions to your tank are also going to have to be considered before adding fish to your tank. Fish will still need ample room to swim around; some fish prefer having more room than others. 

The Plants You Place In Your Tank 

Some plants are better suited to a tank of this size than others. You’ll also have to consider the specific fish you want to live in your tank when selecting your plants. Some of the best plants for 20-gallon tanks include java ferns, water wisteria, amazon swords, and Vallisneria, but there are many more you can choose from. 

How Territorial Your Chosen Fish Are

You’ll have to consider how territorial your chosen fishes are and whether or not they are happy having other fish in their tank. When trying to mix different kinds of fish, where they swim in the tank and what other types of fish they like or don’t like will need to be considered. 

Best Fish To Keep In A 20 Gallon Tank 

There are many types of fish that can be happy in a 20-gallon tank. What will determine their suitability in a tank this size will be how many fish you want to group together. Some fish work well in a tank with multiple fish, while others do not. 


Guppies are perhaps one of the most popular fish for home aquariums. You can keep multiple guppies in a tank that is 20 gallons, but it’s recommended that you don’t go over 10 or 12 at the most. Be sure you keep more females than males in a tank, especially if you don’t want them breeding a lot. 


Tetras are tiny and colorful fish many love having in home tanks, as these fish love being in tanks with other tetras. Tetras are also fairly low maintenance in terms of fish. Since these fish only grow to be less than two inches, you’re able to keep quite a few in a 20-gallon tank, but you shouldn’t keep more than 18 or 19 maximum. 


A molly is a gorgeous fish with distinct, colorful spots and long fins. One thing to be cognizant of when setting up your tank with mollies is that they can breed fast. An ideal setup in a 20-gallon tank would be three female mollies and one male molly. There are various types of mollies you can choose from. 


Barbs can make great tank mates with each other in a 20-gallon tank. You have to be careful with barbs and other types of fish, however, as barbs are known for aggressive tendencies. You shouldn’t add more than six barbs to a 20-gallon tank. 


Zebrafish make for wonderful, small fish that can live happily in a tank of this capacity. It’s also recommended that at least six zebrafish are kept together in a tank, as these fish like having other fish to interact with. They are quite active swimmers as well, making them a joy to watch. 

Pygmy Cory Catfish 

These tiny, partially translucent fish are social and peaceful little fish that can live fulfilled lives in a tank this size. It’s recommended that you pair at least six to eight in a tank together, but don’t add more than this. Pygmy cory catfish shouldn’t be by themselves.


Rasboras are tiny fish that are an absolute delight to watch swim from side to side in a tank. You should have at least six of these fish in a 20-gallon tank to maintain their social needs. They can grow up to two inches long, so you should only keep between 10 to 14 in one tank at the very most, so they have room to swim. 


Corydoras are unique-looking fish that love hanging out at the bottom of the tank, scrounging for food. Corydoras thrive when they are kept together in a tank with a minimum of six other corydoras. 


Danios also come in various subtypes and are known to be a nice, calm fish content living in an aquarium of this capacity. Moreover, they also live well with many other fish of their stature. Danios like living in a community, so it’s ideal that you set them up with a few other danios or other suitable nano fish. 

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What Is The Biggest Fish You Can Put In A 20 Gallon Tank?

Generally speaking, you shouldn’t have fish that grow more than three inches in a 20-gallon tank if you want multiple in one tank. Otherwise, you may be able to go a bit bigger if the fish is able to live on their own or with one other fish. That said, the one-inch per gallon of water rule won’t always apply depending on your choice of fish. 


Cichlid is a general term for a variety of fish, each falling into this classification. Many of these fish are small enough that they can live happily in a 20-gallon tank. Depending on the specific cichlid you choose, you may be limited to only having a few in a tank this size. 


There are a few types of gourami fish that can grow to multiple lengths, and only some of these are suited for a 20-gallon tank. That said, the type of gourami you want to house in your tank will determine how many you can group together. 


The platyfish are a nano fish, but some types of these fish can grow to be about three inches long. Since they are social fish that can live with other fish in a 20-gallon tank, it’s not recommended you opt for a mixture of these fish that grow much bigger than three inches. 

Kuhli Loach 

A Kuhli loach is technically not a very large fish, as they typically only grow to be about four inches long. As such, you should only consider about two to three of these fish in a 20-gallon tank at the most. 


Swordtails are some really beautiful fish, and they get their namesake due to the shapes of their tails. You can have more than one swordtail fit comfortably in a 20-gallon tank, but you should only have about two to three at the most. As such, having two females together, or two females and one male is ideal. 


Plecos are another fish that come with variety, meaning you can find different types of plecos that have their own unique physical and personality characteristics. Since these fish can grow up to six inches long as adults, it’s best to stick with two in a tank this size. 

Do Not Exceed Fish Bigger Than Six Inches 

Six inches is the absolute biggest fish that may be able to live a happy life in a 20-gallon tank. In fact, some might even say you shouldn’t go bigger than 4 inches, depending on the kind of fish you select. The bigger the fish you choose, the less you can have to live in one tank. 

What Fish Go Well Together In A 20 Gallon Tank?

Through much trial and error, some types of fish have been found to live well together in a 20-gallon tank. This is especially useful to know if you enjoy different kinds of fish but don’t want to make any of them unhappy by keeping them in the same home. 

Bettas And Pygmy Cory Catfish 

An ideal setup for this combination of fish would be one betta and a few pygmy cory catfish. These fish may interact somewhat but aren’t likely to scare each other or start fighting with each other. 

Tetras And Guppies 

Even though tetras and guppies are going to want to share space in a tank, they generally get along well with each other since they are both very peaceful fish. These two nano fish are social beings, and as such, they get along well with other nanos. 

Barbs And Mollies 

Barbs and mollies can live peacefully in a 20-gallon tank together so long as it’s not overcrowded. Barbs don’t live well with some other kinds of fish, so mollies are a perfect choice. 

Tetras And Pygmy Corydoras 

These two types of fish can live harmoniously in a 20-gallon tank together. This is because they tend to occupy different parts of the tank, and so they likely won’t interact that much. 

Guppies, Mollies, And Platies 

This combination of nano fish makes for a vibrant and colorful fish tank. All three of these particular types of fish are peaceful and can get along swimmingly together. 

Tetras And Cherry Shrimp 

Tetras tend to swim near the top or middle of a tank, while cherry shrimp and other types of fish stick to the bottom. Having shrimp in an aquarium is also nice to help keep the tank clean. 

Swordtail And Guppies Or Tetras

You can pair one or two swordtail fish together with a few guppies or tetras. They are all generally calm fish, so they are likely to get along quite well and will share space nicely. Additionally, this would make for a visually striking tank. 

What Fish Shouldn’t You Keep In A 20 Gallon Tank?

Not all fish are suited to a 20-gallon tank, even if they were to live as solitary fish. Furthermore, if you’re looking to have a mixture of fish in a tank of this size, there are parameters to consider before you start designing your aquarium. 

Don’t Mix Predators And Prey 

This should be a given, but it’s important to know which fish are at risk of being harmed, harassed, or even fatally injured by other types of fish. These fish shouldn’t be put together in a tank of any size, as it would be very cruel. 

Fish Who Live In Different Water Temperatures

If you are mixing some types of fish together, you need to be certain that they can survive within the same water parameters. This includes the same water temperature, as well as with the same types of tank setups, pH levels, filtration systems, and currents, among other factors. 

Fish Who Produce A Lot Of Waste 

Some fish are more prone than others to make a lot of waste, which can contaminate water quickly if it either can’t be filtered out or can’t be cleaned in time. Fish such as these, even if they are small, will need a larger tank to ensure fish can’t get sick from large quantities of waste. 

Large Fish

Some fish grow to be way too big to live comfortably in a 20-gallon tank. Fish need space to swim around and also need space for entertainment, plants, and decorations in their tank. Even if some fish are smaller as babies, a 20-gallon tank should never be considered a suitable living space for them. 

Highly Territorial Fish 

Twenty gallons is not a lot of space for fish that are territorial to get away from each other if need be. Even if you set up ample hiding spots for fish to retreat to, it doesn’t guarantee that territorial fish put together in a confined space won’t have detrimental consequences. 

Fix That Grow Above Six Inches

As mentioned, you should only be keeping small fish in a 20-gallon tank, even if you plan on only having one fish live on their own. Even one fish can feel cramped and overcrowded in a tank that’s too small for them, elevating their stress levels to the point where they can get sick. 

Fish That Breed A Lot

When you have a male and female fish of the same type together, there is only so much you can do to prevent nature from running its course. 

Frequently Asked Questions 

It’s evident that there are numerous items to consider before purchasing a tank, and once that’s done, there are even more considerations to be made to set it up. If you’ve made your mind up about choosing a 20-gallon tank, knowing some of the most commonly asked questions about setting up a tank of this size is important. 

How Many Fish Can You Keep In A 20 Gallon Saltwater Tank?

The type of fish you choose to keep in a saltwater tank will dictate how many are safe to keep in one that is 20 gallons. Generally, you shouldn’t be keeping more than six or seven fish in a tank this size when it comes to a saltwater tank.

 It’s important to ensure that any fish you keep in a 20-gallon saltwater tank doesn’t exceed four inches in length as adults. 

How Many Betta Fish Can You Have In A 20 Gallon Tank?

You won’t be able to fit multiple betta fish in a 20-gallon tank and have them remain happy. If you want bettas at home, stick with having three female bettas in a 20-gallon tank, or have just one male betta. Mixing multiple bettas together can cause them to fight over territory, and you’ll likely end up losing at least one of them. 

How Do You Stock A 20 Gallon Fish Tank?

Stocking a 20-gallon fish tank is best done by pairing nano fish together. A nano fish is classified as a fish that only grows a maximum of two inches long, and many of these particular fish are peaceful species that can get along with other nano fish. 

You should stick with under 10-16 or 18 of these nano fish in a 20-gallon tank, decreasing the amount of fish based on maximum size and temperament. You should give them ample room to swim around and enjoy themselves. 


A 20-gallon tank, while not especially large, is a great size for someone who is just getting to learn about the aquatic world and is getting used to having a tank at home. Tanks this size can accommodate multiple fish in certain situations, especially if you are looking to intermingle different types of nano fish. 

You want to account for what else is going to take up space inside your tank before determining exactly how many fish you’re going to keep in this size tank. In order for fish to be happy in their tank, they need space for swimming, sleeping, eating, and playing. When all of these factors are accounted for, a tank this small can be home to a beautiful aquatic habitat.