How Many Fish Can Live In a 20 Gallon Tank?

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How Many Fish can live in a 20 gallon Tank

How Many Fish Can Live In a 20 Gallon Tank? While there is no exact number of fish to be recommended, there are a few factors to be considered that can help to decide the optimum number of fish for your tank.

20-gallon aquariums are known to be perfect for beginners who prefer to start small but not only for amateurs since, in some instances, this tank would be of the most decent size for some species of fish. While this tank size would be considered a ‘small aquarium,’ it is possible to home a variety of live plants and fish healthily.

To create a lively environment in your 20-gallon tank, I will share some key points based on my experience and extensive research for you to make the perfect choices when selecting the number of fish for your tank. Below are the topics focused on in details:

  • Best fish for a 20-gallon tank
  • Biggest fish for a 20- gallon tank
  • 10 most popular fish that can be compatible with a 20-gallon tank
  • Other factors to consider
  • Compatibility
  • Types of water
  • Maintenance required
  • Types and volumes of plants
  • Types and volumes of decorations

How Many Fish Can Live In a 20 Gallon Tank? There are Many Factors to Consider!

For a tank of this capacity, it is recommended to keep relatively small fish. Typically, fish that do not exceed a size of 6 inches are appropriate but are subject to a few other key factors, which I will further discuss in detail. A common query of beginner aquarists is whether a 20-gallon tank is sufficient to hold any fish. The answer is that you will find it difficult when selecting your favorites from the wide range of ornamental fish breeds suitable for a tank this size! Read on for a review of all the factors to consider.

Biggest Fish for A 20-Gallon Tank


Goldfish can grow over 5 inches long, requiring sufficient space for healthy growth. For a tank of this size, a fully grown pair can be housed but not any more than that. However, it is possible to keep up to 4 small goldfish for short periods. With time, the small goldfish should be relocated into a larger tank to prevent stunted growth and stress. So ideally, a maximum of 2 adult goldfish can be kept in a 20-gallon tank, but it could be subjected to its size as some goldfish could grow more significantly, which may require a bigger tank.

Dwarf puffer fish/ pea puffer

Freshwater dwarf puffer fish can be a suitable species for a 20-gallon tank. Up to 4 pea puffers can be kept in a tank of this size. However, it is essential to note that other varieties of puffer fish that tend to grow much larger cannot be housed in a 20-gallon tank unless they are juveniles.

Cory catfish

Growing up to four inches long, cory catfish are another species that can be kept in a 20-gallon aquarium. The recommended quantity is 2 pairs of these fish in a tank of the above size. Since they are known to be a schooling breed of fish, keeping at least 4 fish together is suggested, yet the more, the better. Nevertheless, for a 20-gallon tank, it is recommended to stock only a small number of these fish.

Kuhli loach

Although kuhli loaches are bottom dwellers and relatively inactive, keeping a maximum of 2 fish in a 20-gallon tank is sensible. This is because it is their nature to scavenge around the surface for food, and placing them in a small tank would likely cause stress. Furthermore, it is inexpedient to keep just a single kuhli loach due to uncertainty of the tank size due to their social behavior. Therefore, for a 20-gallon tank, you can house 2 kuhli loaches without any concerns.

Upside down catfish

Upside down catfish is a species with a calm temperament that can grow up to 4 inches in size. They can be a great addition to a 20-gallon tank when kept as a group of 2 fish. Even though it seems possible to put more of these fish into the aquarium, it could affect their growth and behavior, which is not something aquarists look forward to.

Paradise fish

Paradise fish are another breed of fish that can be kept in a 20-gallon tank. Only a single fish can be housed in a tank of this size as they are territorial and may attack other fish. It may not be pleasing to keep a single fish in a tank, but when choosing a tank of this volume, it is wise to keep them alone. If intending to add more fish of this species, it is best to go for a larger aquarium that gives adequate space, reducing the possibility of aggression.

Blue Gularis killifish

This tank size is often considered minimum when housing blue gularis killifish. This is simply because of their dynamic nature, which requires ample space. If provided with a smaller tank, they are likely to jump out. Up to 2 fish can be kept together in a tank of this size, and if you are planning on getting more of these fish, it is advisable to get a larger tank.

Freshwater/ African butterflyfish

If planning to add just a single or a pair of African butterflyfish, a 20-gallon tank will be suitable. These fish can reach up to a size of 5 inches, and their temperament suits this type of tank. Since they are air breathers, it is okay to have larger aquariums to have more oxygenated waters. However, these fish are not commonly kept in small tanks as aquarists prefer having them in larger tanks with more company.

The above list of fish mentions some of the biggest fish that can be kept in a 20-gallon tank. However, considering their growth and temperaments, they may only sometimes be popular to be featured in tanks of this size.

Below are the top 10 popular fish appropriate for a 20-gallon tank. If you are planning on jumping into this hobby, consider getting one of the following kinds of fish because they do well in a tank this size.

10 Most Popular Fish That Can Be Compatible For A 20-Gallon Tank

Guppy fish

Guppies come in various vibrant colors and will also be an excellent species for your tank. Guppies are small and have a calm temperament, making a 20-gallon tank a suitable environment for them. They tend to swim close to the water’s surface, which implies that it is optional to have big tanks with huge depths. Try not to exceed housing more than 14 guppies per tank, as it would result in overcrowding and stress.

Tetra fish

Whether it is neon tetra/cardinal/ glow tetra, rummy nose tetras, or even a mix of them, they are all ideal fish for this tank size. Aquarists often build decorative planted tanks and keep tetras with them to give a stunning combination of colors. Tetras are known to stay together in schools, so it is ideal for keeping about 20 fish, allowing them to form a decent-sized group without fearing overcrowding your tank.

Male betta/ Siamese fighter fish

Some fish keepers may argue that male betta fish do not need a 20-gallon tank as they are small in size, and a 10-gallon tank would possibly be the maximum. When providing a larger tank, the healthier it is for bettas, but it should be shallow enough as they are air breathers and have to come to the surface to grasp oxygen regularly. Bettas may not be very active and might not visibly make maximum use of the tank, but large tanks offer steady water conditions.

Platy fish

Platies are another great addition, as they bring much-needed energy to your aquarium. Platies come in various colors and patterns, making it possible to add a vivid variety of fish by selecting and matching your favorites from their available sorts. Platies may often engage in breeding, so it can be either good or bad, depending on your preference. Nevertheless, it is possible to prevent the breeding of platies in your tank by placing only male fish. If the reproduction of platies is not a concern, you can feel free to add up to 10 pairs of platy fish.

Dwarf gourami

Dwarf gourami is some of the most social fish species you can have, which is probably one reason for their popularity. You can keep 5-6 dwarf gourami in a 20-gallon tank without any worries when provided with a sound filtration system. There are different colors and patterns of dwarf gourami, allowing you to select the ones with your favorite colors and designs. An assorted range of these fish would look lovely for your tank, and they are undoubtedly marvelous creatures to care for!

Zebra fish/ Danios

Known for their vibrance and energy, danios are some of the favorites for small tanks. Growing less than 2 inches, these fish are excellent for planted tanks, and their known aggressive behavior allows them to be kept with many other species of fish. Electric green, pink, yellow, blue, red, and brown are some of the most commonly available colors. For a tank of this size, it is ideal for keeping up to 14 danios, and they often stay in schools, so it is best to keep a significant number of them together.  

Female betta fish

While it is convincible that female betta fish may look less enticing than male fish, females can be kept together, which helps build a beautiful aquarium. Females may often show signs of aggression by chasing fellow tank mates, but they do not attack each other, unlike the males. This tank size will be ideal for setting up a betta sorority of not more than 6 fish. Female bettas can be a fun species to have; some have modern color patterns and designs due to selective breeding.


Another small yet vivid type of fish that would do well in this tank size are mollies. Growing up to 4 inches long, these fish are popular with aquarists who maintain planted aquariums. Mollies are quick breeders as they are likely to reproduce whenever given a healthy environment. Furthermore, there are varieties of molly fish that differ in design and color. Colors range from yellow, orange, white, and black, while some fish have colorful patterns.


Rasboras are schooling fish and are less likely to be stressed when kept in groups. Therefore, keeping around 15 fish together in a 20-gallon tank is ideal. They generally have a peaceful temperament, but they are undoubtedly fast swimmers and make use of every square inch of the tank. They are commonly featured in community tanks as they pose no harm to other tank mates. These fish are ideal for beginners though loved by aquarists of all levels.


Freshwater shrimp are well known amongst fish keepers, and this tank size would be perfect to house many of them! Usually growing up to an inch long, shrimps are colorful, and they never fail to make their tank look magical. There are different types of freshwater shrimp, namely, ghost shrimp, cherry shrimp, bamboo shrimp, and chocolate shrimp. When provided with adequate surface area by placing plants, rocks, wood branches, and decorations, more shrimp could be kept in the tank as they do not swim but move on surfaces. Adding about 40 shrimps per tank of this specific size is recommended.

The above list includes some of the most prominent fish that are kept in a tank of 20-gallons in no particular order. Numerous varieties of fish are suitable for these types of tanks, but only a few of the most loved fish have been mentioned.

Factors To Be Considered

There are a few key factors to consider when deciding what kind of fish you want to keep in an aquarium this size. Simply placing small fish into a small aquarium would not be sensible. Likewise, to ensure that your fish are living a healthy life, it is crucial to identify these factors and decide on the most appropriate decisions when it comes to choosing fish. 


It should be evident that fish need ample space for moving around, just like any other animal. Housing relatively large fish in small tanks is essentially inhumane because they are likely to feel crammed in a tight space. The standard measure of thumb for calculating the number of fish in a tank is 1 inch of fish per single gallon. This measurement seems to be mechanical because, for example, a flowerhorn of 8 inches would require a tank of 8 gallons according to this measurement, which is unrealistic. However, this method is more effective when used on smaller fish that do not exceed a size 3 inches.  

It is essential to take growth size into account since little fish have the potential to get larger, necessitating a larger habitat. For instance, a juvenile discus fish would find a 20-gallon tank sufficient, but along with its growth, the discus fish should be relocated to a bigger tank. This is because fish physically enlarge while needing more space. Hence, when selecting fish for your 20-gallon tank, it is suggested to pick fish that do not grow progressively into giant creatures. It is recommended to choose fish that reach a maximum length of 5 inches as they have lesser needs of being transferred into much bigger tanks.


Another critical factor is the behavior of the fish which you choose to get for your tank. The nature of fish often depicts the aquarium size which suits them, so it is fundamental to research and improvise your knowledge of the behavioral patterns of fish you prefer to have.

For instance, angel fish may seem suitable for a 20-gallon tank due to their size. However, their temperament causes them to chase and attack other angels when housed in a tank of this size. This makes it clear that, to house more than a single angel fish, it is always a good decision to invest in a larger tank. Likewise, this applies to other types of fish and is not just limited to angels.  

Moreover, fish have unique swimming patterns. For example, bettas share a different swimming pattern with goldfish as bettas tend to stay constant at most times while goldfish are always on the move. Considering this point, it is incorrect to house quick and consistent swimmers in a small tank. Doing this will cause stress for your fish and may result in a decline in their lifespan.


Generally, some types of fish produce more waste compared to others. Despite this factor, if you wish to keep these fish, you can overcome this issue by fitting an appropriate filtration system. Especially in small tanks like 20-gallons, fish waste could contaminate the water rapidly. Therefore, it is essential to clean it or have a filter. Mollies are an excellent example of fish that are known to be messy, so if you do not have a filter in the aquarium, they are likely to cause dirty water.

If you do not have access to a filtration system and find it overwhelming to conduct water changes routinely, fish such as neon tetras, swordtails and danios are more ideal to be considered as they typically produce less waste.


Community tanks have a growing popularity within the aquarium industry as they display a diverse range of fish, making them appealing. For those who don’t know what a community tank is, it is an aquarium with various fish species. The challenge of forming a community tank is identifying and pairing fish without causing any aggression or fighting. It is best to conduct research before purchasing any fish to prevent your tank from being a battlefield. Following are some exemplary fish combinations you could try out:

Dwarf gourami is territorial and may attack fish unable to defend themselves. Therefore, it is vital to add only fish compatible with dwarf gourami when building a community tank featuring them. Fish such as rosy barb, pleco, harlequin rasbora, and Corydoras are suitable tank mates for gourami as they are unlikely to get attacked. Another fantastic combination for gourami is by adding an assorted range of species, such as: chocolate gourami, pearl gourami, honey gourami, and flame gourami together.

This is undoubtedly the most desired combination of fish in 20-gallon tanks, which consists of guppies, danios, platies, mollies, tetras, barbs, or any other small-sized peaceful fish. These fish are all vibrant in color, and by having a mix of them, your aquarium will display a mind-blowing appearance that you can gaze at for literally hours. This combination prevents making your tank look dull as guppies prefer swimming close to the surface while the other species enjoy staying everywhere, which means that the whole tank is visibly occupied! A common mistake made by aquarists is overstocking their aquarium because they get carried away with their beauty.

If you wish to aquascape your 20-gallon tank and make it a planted aquarium, it is best to add fish that do not cause any harm to the plants. Many aquarists who are new to planted aquariums learn their mistakes by the hard way, which is after the fish have destructed the carefully fitted settings. Shrimps, mystery snails, tetras, dwarf puffer fish, and killifish are certain breeds of fish that would not ruin the plants in the aquarium. It is important to note that selecting fish suitable only for planted tanks would not be sufficient as their aggression towards tank mates is also a valid concern to consider.

Moving on, a female betta sorority tank could be formed by introducing a suitable number of female fighter fish together. If planning on keeping female betta fish with a different species, selection should be made wisely as female bettas may attempt to chase or nibble on other fish. A few types of ideal tank mates are; snails, tetras, platies, and plecos.

Types Of Water


Freshwater is the most popular type of water used in fish tanks due to its ease of access. Water for freshwater tanks could be sourced by municipal lines, wells, rain collectors, or reverse osmosis systems. Each of these sources has its own pros and cons, as some lack minerals, and others contain chemicals such as chlorine used for purification. The above listed fish are all suitable for freshwater aquariums. Different fish require various water parameters, but in general, a pH of 6.5 to 7.5 and a nitrate value below 25ppm is considered ideal.  


Saltwater tanks are comparatively less popular than freshwater tanks as they can be more complex and costlier to maintain. Saltwater is made by mixing specific measurements of reef salt with fresh water. Unlike freshwater fish, saltwater species tend to require more space. However, there won’t be many options for fish in a 20-gallon tank, unlike in freshwater tanks. Clownfish, dartfish, and watchman goby are some ideal fish for saltwater tanks of this size.

Brackish water

Brackish water refers to a combination of fresh and salty water. An advantage of having brackish water is that they do not sustain most diseases and parasites, which makes it a healthy environment for fish. Most experienced aquarists prefer brackish water because it helps to keep their pets healthy, and it is also economical as you do not have to spend much on medicine and treatments. Some species compatible with a 20-gallon brackish tank are; mollies, puffer fish, guppies, and swordtails.

Maintenance Required

It is essential to keep your 20-gallon tank tidy at all times in order to prevent your fish from getting various diseases. Since the tank is relatively small in size, the water is prone to get worse much faster. If there is a proper filter in place, the tank would require less maintenance, so if you have a busy schedule and cannot commit yourself to perform regular water changes, a filter would be helpful.

When tanks are overcrowded, they require more maintenance because the more fish are housed, the more waste they produce. If you fear your tank gets contaminated faster, this could be a potential reason. To overcome this problem, relocate some of your fish into another aquarium, and this would lower the number of water changes needed to be done.

Another probable reason could be due to overfeeding. When feeding more than what the fish could consume, leftover pellets or food would dissolve in the water, which could discolor the water and create a bad odor. This is a common mistake by aquarists and would end up deadly in certain instances, as ammonia could rise due to impurities. To overcome this complication, make sure only to feed a consumable amount of food for your fish and then scoop out if any excess food is lying around. Another solution for this is introducing scavengers such as snails, shrimps, and plecos who devour any leftovers.

When it comes to performing water changes, if your tank has a filter in place, it is recommended to do a 20% water change once every two weeks and a clean-up in the filter once a month. Whereas, if your tank does not have a filter, make sure to do the same but once a week in order to maintain a healthy aquatic environment.

Some aquarists state that densely planted aquariums with fewer fish do not need a filter as live plants absorb harmful impurities such as ammonia. While this is true to a certain extent, plants are incapable of soaking up fish waste and other debris. However, if you siphon out any visible debris and perform timely maintenance, it is possible for such planted tanks to run without a filter. If your tank comes in contact with direct sunlight or sunrays, you may able to see algae on the glass, which look like tiny green dots. Scavenger fish may not be able to completely clear of these, so a magnetic brush or a sponge could be used to scrub off such algae as they may disturb the view of the aquarium.


Types And Volumes of Plants

Adding live plants to your fish tank has numerous benefits for your loving fish. A few of them are; they release oxygen, help to clear ammonia and nitrates and promote the growth of beneficial bacteria. When it comes to the volume of plants, it would depend on the number of fish you wish to have. Especially in a 20-gallon tank, adding plants may take up a significant amount of space, so you should be mindful of this. If you are planning on having a few small fish, more plants can be planted, and vice versa. Following are some plants ideal for a 20-gallon tank;

Pygmy chain sword

This type of plant is relatively easy to grow and reaches a maximum of 5 inches. Its hardy nature makes it ideal for beginners with less experience in planted tanks.

Guppy grass

Your choice will determine whether you plant guppy grass or let it float. They can grow quite fast, and it supplies ample hiding space for fish and fry.

  1. Marimo ball

This is a type of algae that forms into a shape of a ball. Growing to a maximum of 3 inches, they do not take much space in your tank, and they look pretty unique, which adds up to the beauty of your tank.

  • Micro crypt

Reaching a maximum size of 5 inches, this is almost a dwarf version of other crypt varieties. Native to the crystal waters of Sri Lanka, they are compatible with aquariums with various water conditions.

  • Dwarf Hairgrass 

This particular plant is a favorite amongst many in this hobby due to its stunning appearance. Ever seen aquariums with green grass on the bottom? If yes, it probably would have been dwarf hair grass. This is perfect for planted aquariums as they offer a beautiful green touch to the depths.


Types And Volumes of Decorations

One of the most entertaining parts of this hobby is when decorating your fish tank. This is the time for you to unleash your creativity and build up an alluring aquarium for you to enjoy looking at. Similarly, with aquatic plants, it is best to stay away from adding an overwhelming volume of decorations in a 20-gallon tank as they take up space for your fish. Following are some decorations that you can contemplate getting;


Rocks are widely used by aquarists to decorate tanks. For creatures like shrimp and snails, rocks will provide excellent surface area. Pet stores offer stones of different sizes and colors for you to decorate the tank with. They also offer plenty of hiding spots for fish.


Branches are prevalent in almost all planted and non-planted aquariums due to the unique appearance it provides. Placing branches allows your aquarium to resemble a natural environment by visibility and also provides a surface for algae growth. It is crucial to add sterilized wood branches available at pet stores as some could lose color while affecting the water quality. A few popular types of wood which you could get for your aquarium are; Malaysian driftwood, Azalea root, and spider wood.


Inserting ornaments also generates hiding spots for your fish which could assist them when defending from other species and keeping themselves calm. For a 20-gallon tank, it is advisable to add more miniature decorative ornaments so that your fish have adequate space to swim around. The primary purpose of adding ornaments is to make your aquarium a captivating one, so make sure to enjoy yourselves when doing this exciting task!

Wrap Up Thoughts

A 20-gallon fish tank has so much to offer, and this is never too small to have an enjoyable experience. If you follow the above guidelines and ways are being followed, you can make this tank a great aquatic environment for your loving fish. I believe that the most crucial factor when having a tank of this size is selecting the ideal type of fish. This article covers this particular segment, and if you have any doubts regarding the compatibility of a particular fish, it is highly advisable to do research or consult a knowledgeable individual in this field.


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