How Many Fish in a 40 Gallon Tank? – Fish Raising Tips

how many fish in a 40 gallon tank

A 40-gallon tank is one of the most popular choices for beginning aquarists since it can house a wide variety of species. This type of tank (with a reef-inspired design) may accommodate up to 25 small fish.

However, if you seek bigger fish or more decorations, the capable number will drop substantially. Therefore, you should take a look at my suggestions below and find out the answers to your question “how many fish in a 40 gallon tank”.

Fish Types and Their Suggested Population in a 40 Gallon Tank

As I mentioned, depending on the size and the type of fish, you can accommodate more or less compared to my suggested number of 25. In order to make things clearer, here is a table for you:

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Fish TypesNumberFeatures Must-considered Issues
Goldfish Maximum 2Cheap, a friendly species, improve owners’ creativity, live quite long (about 20-30 years)Need a large-enough tank with proper filter, have sensitive digestive system, about to pollute their living area
Mollies15 to 20A great algae-cleaners; have beautiful appearanceMale mollies are extra aggressive
Angelfish Around 4 to 6Long lifespan (10 years), have many bright colors, easy to get sickQuite big (around 10 inches tall and 6 inches long), violent and mean when having stress, easy to get sick
DaniosIt depends (*)Don’t require large tank, non-picky, very popularA typical schooling fish,
GuppiesAbout 20 to 30Social fish, easy to take care, fit well with a community tank, can live in both freshwater and saltwaterRequire completely clear water, take more effort to maintain their living condition
BettasAround 15 to 20Appealing outlook, good for decorating your tank, low-maintenanceProne to violent
Clownfish2 (adult-sized, coupled) or only 1A saltwater fish, good option for breeding fishShould go in a reef-inspired tank
GouramiAround 10Friendly fish; a kind of exotic freshwater fish with outstanding lookSensitive with sudden changes in living environment (some kinds, such as honey gourami)
Bristlenose PlecoOnly 1Adaptable, gentle, straightforward to raise, a safe start for novice aquaristBecome fierce in the breeding season
DiscusAbout 4A freshwater fish, safe for community tankPretty hard to keep with beginners
CichlidsAbout 6 (medium-sized) to 12 (dwarf-sized)Easy to maintain, has an extensive/easy-to-find menu, can change colorsPossible to have fatal diseases (bacterial infections, white spot, skin sores) if the water condition is unstable
Cory Catfish/CorydorasAt least 6 to 10Help cleaning their living environment, easy-going, highly-adaptableSusceptible to deadly disease including ich or white spot
TetrasAround 20 to 30A friendly breed, adaptable, quite hardyRequire more attention to maintain their living conditions, especially in breeding tank
ChromisAround 3Live long (around 8 to 15 years), attractive look, easy to raiseCannot eating to much, be careful when feeding them
Tiger Barbs8 to 10Looking vibrant, low maintenance, active-swimmersA bit aggressive, not fit for a community tank


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  • Because there are so many distinct types of danios, it’s hard to predict their popularity in a 40-gallon tank. For example, while you can have up to 18 little spotted danios (just 2 inches long), 4 Bengal danios are the perfect quantity (6 inches long).


Meanwhile, you may keep up to 12 medium-sized danios, such as zebra danios, in this type of aquarium.

Overall, each of the fish species depicted above is a good pick (with its own set of pros and cons) for a 40-gallon tank that you should never underestimate.

Setup Ideas: Suggested Fish for 40 Gallon Tank


If you’re still stumped after looking over my proposed table above, I’ll clear you out with this section of 40 gallon tank stocking ideas.

Suggestion 1:

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Species Number of fish
Bolivian Rams2
Sterbai Corys6
Neon Tetra10


Suggestion 2:

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Species Number of fish
Celestial Pearl Danios8


Suggestion 3:

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Species Number of fish
Bristle Nose Pleco1
Amano Shrimp4
Rummy Nose Tetra5
Cory Catfish4

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Don’t be worried about Amano Shrimp because they are very adaptable with any tank environment. They are diligent cleaning workers with a simple diet of leftovers and algae.

Furthermore, they can peacefully coexist with other fish, making them an excellent choice in this setting.

Suggestion 4:

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Species Number of fish
Bristle Nose Pleco1
Neon Tetra5
Dwarf Gourami1
Zebra Danio5


Otherwise, creating a tank with only one type of fish is also great. I promise that looking at gorgeous, sparkly mollies in a 40-gallon tank (or bettas, goldfish, or cichlids) will bring you delight.

Things to Consider Before You Stock Up on Your Tank


Fish Size:

Rule number 1, big fish eat small fish, and you should notice that your beloved pet may end up in the other ones’ mouth. Therefore, attempt to have fish at a similar size in the same tank.

Number Of Fish:

Aside from the maximum quantity of fish can be in a 40-gallon tank and the practical 40-gallon fish tank ideas, you can also utilize the fish calculator below to design your own fish community.

With my recommendation here, you will calculate the amount of fish based on their adult-sized size. More detailedly, with every inch length of the fish, you should have at least 2 gallons of water. With an 8-inch fish, you can have 2 of them in your 40-gallon tank.

Remember, this suggestion works better with big fish, so don’t hesitate to apply it once you have a chance.

The Compatibility Between All the Fish in Your Tank:

Before you start building your own aquarium, do a little research or refer to my table above to understand more about the habitat of your chosen species. Multiple kinds are hostile and inappropriate for a diverse society.

For instance, you should never put betta fish or tiger barbs with the other friendly kinds. In the case of betta fish, the male ones are somewhat more aggressive, that you can’t even let them stay together.

Luckily, every problem can be solved. Even if you are paying interest on bettas, a type of fish with renowned aggressive characteristics, you can 100% build a community of them once you learn the tip to ease their stress by letting them (especially male ones) alone.

Water Preparation (pH Level, Filtration System):

Each variety of fish has diverse needs for their living habitats; thus, it is essential to adhere to them in order to assist your fish in living a healthy and happy life.

In detail, while guppies and gouramis need clear water to survive, tiger barbs are easier to care for.

For the filter, you’ll want to pick a nice one that’s appropriate for your 40 gallon tank. Choose one of these based on your demands.

Frequently Asked Questions


Is a 40 gallon aquarium big? 

Yes, it’s big, but not too huge. Imagine it has the same size as a medium fridge. There are two main kinds of 40 gal tank: normal (with rectangular bottom) and breeder (with square bottom). Both are usable, but I suggest you employ the breeder if you are a novice.

How often do you clean your 40 gallon tank?

It depends. Normally, it is recommended to clean your fish tank once or twice a week. However, this job also depends on the tank’s condition. If your fish don’t produce much waste and your filtration system is excellent, you may clean it once a month.

Which kind of fish cannot live in a 40 gal tank?

The answer includes flowerhorn and oscar. They might be the coolest fish to think of, but they are also one of the biggest fish to raise in your home aquarium.

If you wish to keep an adult flowerhorn (which is around 12 inches long) healthy and happy, you should at least give them a 200-gallon tank (or up to 375).

Same to oscar, this kind of fish needs a tank of around 55 to 100 gallons for an average, adult-sized individual of 10 inches long.


If you’re looking for 40 gallon freshwater tank ideas, thinking about gourami or danios is never a poor decision. But unless your intended homemade aquarium is a 40 gallon community tank, tetras or guppies are the ideal candidates.

Otherwise, considering their longevity before adopting them is another approach. In this circumstance, Chromis, goldfish, or angelfish are strongly recommended.

To sum up, feel free to seek out various types of fish based on your preferences. No need to wonder “how many fish in a 40 gallon tank” anymore, and allow your creativity to soar!