Information, Aquariums

How Many Fish in 40 Gallon Tank: Here’s the Ultimate Guide!

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by Jason Matthews



Overpopulating a fish tank is always a recipe for disaster. It can cause an overall decrease in the oxygen concentration due to increased waste, leading to the death of many fish in the tank. So, what to do if you have a 40-gallon tank? How many fish can you keep in it?

On average, you can keep 15 to 20 small-sized fish in a 40-gallon tank. However, for large-sized or medium-sized fish, you need more swimming space. So, you should keep up to 3 to 4 medium-sized and 1 to 2 large-sized fish in a 40-gallon tank.

If you wish to know more about taking care of your fish in a 40-gallon tank, read on!

Key Takeaways:

  • A 40-gallon tank is a beginner-friendly one best for small and medium-sized fish species.
  • As per the general rule of 1-inch fish per 1 gallon, you can keep 15-20 small-sized fish.
  • For medium or large-sized fish, keep only 3 to 4 (medium-sized) and 1 to 2 (large-sized) fish. 
  • Guppies, tetras, mollies, and gouramis are some of the best fish species for a 40-gallon tank. 

How Many Fish Can You Keep in a 40 Gallon Tank?

40 gallon fish tank fish

When you’re deciding how many fish to keep in a 40-gallon tank, there are three main factors to consider. These include:

  • Waste management 
  • Swimming space required for each fish 
  • Aggression levels of various fish 

However, you can make things easier by following a general thumb rule. Technically, you can keep 1 inch of fish per 1 gallon of water. Hence, if you’re planning to keep small fish (between 1 to 2 inches), like Guppies or Neon Tetra, you can keep 15 to 20 fish in a 40-gallon tank. 

Although, as per experts, it is best to not follow the one-inch rule for fish larger than 3 to 4 inches. It is because the overall body volume of 10 small fish combined is usually lesser than a large 10-inch fish. So, the latter requires more swimming area. 

Due to the above-mentioned reason, try to limit the number of medium or large-sized fish to 2 to 3 as some fish (like goldfish) can grow up to 10 inches.  

Which Fish to Keep in a 40 Gallon Tank?

Now, you might wonder which fish is suitable for a 40-gallon tank. Well, ideally, it is best to keep small-sized (1-2 inches) to medium-sized (3-6 inches) fish in a 40-gallon tank. 

The table below will give you a clear idea of which fish species to keep and how many in a 40-gallon tank. 

Fish NameNumberBenefits 
Cichlids5 to 6 (mid-sized)10 to 12 (dwarf-sized)Suitable for community tanks due to their adaptive nature.
Tetras20 to 30Friendly and peaceful with most other fish species like Guppies, Mollies, Betta, and Guramis.
Mollies15 to 20Help in keeping the tank clean by feeding on algae.
Betta15 to 20 Offers a beautiful appeal to the tank with its attractive features.
Guppies20 to 30Extremely friendly, low-maintenance, and love to stay in a community.
Gourami10Non-aggressive and get along with most other friendly fish species.
Cory Catfish10Known as “tank cleaners” as they help in cleaning out any leftovers. 
GoldfishOffers various benefits to humans but releases more waste as compared to other fish species.
Clownfish1Easy to care for but requires proper space.

Best Setup Ideas For a 40-Gallon Tank

You need to take note of the temperament of different fish species if you’re planning to set up a proper 40-gallon community tank. But, if you’re feeling lazy, consider some of the below-mentioned setup ideas. 

Setup Idea 1:

Fish SpeciesNumber 
Neon Tetra5
Zebra Danio5
Dwarf Gourami 1
Bristlenose Pleco1

Setup Idea 2:

Fish SpeciesNumber 
Rummy Nose Tetra6
Cory Catfish5
Amano Shrimp5
Bristlenose Pleco1

Setup Idea 3:

Fish SpeciesNumber 
Neon Tetra15
Bolivian Rams2
Sterbai Corys6

Is a 40-Gallon Tank a Good Size?

If you’re planning to purchase a 40-gallon tank, then it’s a good idea. Moreover, it’s also a good size at least for tiny fish species like guppies and tetras. There are also various other benefits to having a 40-gallon tank.

  • Best for Beginners Due to Low Maintenance and Cost

A 40-gallon tank is the right choice for people who are getting started with aquariums. These tanks are quite easy to maintain and clean. Additionally, they don’t cost a fortune – so you can purchase them whenever you want. 

  • Can House Several Fish Species

If you’re interested in keeping small fish species only, then a 40-gallon tank is one of the best sizes. It can house more than 20 small fish species.

Additionally, if you do want to keep a fish like a goldfish or a clownfish, you can keep them in a 40-gallon tank as well.

  • Easily Available for Purchase 

40-gallon tanks are quite popular amongst aquarium owners. Hence, you can easily purchase them and their related equipment without the need for intense searching.

Frequently Asked Questions

How many guppies can I put in a 40-gallon tank?

A 40-gallon tank is excellent for tiny community fish species like guppies. So, you can keep at least 20 guppies in a 40-gallon tank. However, try to ensure that you keep 2 females for every 1 male to maintain peace. 

How many goldfish can live in a 40 gallon tank?

Goldfish release more waste than many other fish species. It is recommended to keep only one to two goldfish in a 40-gallon tank. However, try to opt for a 50-gallon tank for two goldfish.

How often do you clean a 40-gallon tank?

You need to clean your 40-gallon tank at least once in two weeks. 

Final Thoughts

A 40-gallon tank is the right fit for tiny fish species such as tetras, guppies, and many more. As discussed in this blog post, you can keep around 15 to 20 small-sized fish and 1-2 large-sized fish in a 40-gallon tank. 

You can also keep different fish species in a 40-gallon tank after considering factors like the fish’s temperament and swimming space requirements.

We hope this article helped you figure out how many fish and what kind of fish you can keep in a 40-gallon tank!

Featured image credit: Krys Amon, unsplash


Jason Matthews

My name is Jason Matthews, and welcome to my website. When other kids were bragging about how their dog could sit and roll over, I was bragging about my latest Betta Fish and the cool sea castle I just added to his aquarium. 

Jason aquariume

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20 thoughts on “How Many Fish in 40 Gallon Tank: Here’s the Ultimate Guide!”

  1. I just set up a 40 gallon breeder tank in my classroom. Students are hoping for a combination of the following: khuli loach, African dwarf frog, harlequin rasbora, small Cory, female betta, guppy. Is this possible? If so, how many of each?

    • Hi Susan,

      You can keep Khuli Loach, Harlequin Rasbora, Small cory, Female Betta, Guppy, and African Dwarf frog in the same tank. They are compatible tankmates.

      Guppies (5)
      Harlequin Rasbora (5)
      Small Cory (5)
      Betta (1)
      Khuli loach (7)
      African Dwarf Frog (4)

  2. I currently have a 10 gallon aquarium tank with 3 glofish tetra and 3 albino corydoras. I purchased a 40 gallon tank, so I am wondering what other fish can live with glofish tetra and corys and how many? I would love to get guppies, but can they live with glofish tetra? I am pretty much new and every advice would be great to hear. Thank you.

    • Hi Snezhana,

      Guppies can live with Glofish tetra and Corydoras. In a 40-gallon tank, you can have around 8-10 Glofish tetra and 4-6 albino corydoras. You can also add a group of 8-10 guppies, bringing the total fish count to around 20-25.

      It’s important to note that the fish you add to your tank should have similar water parameters and temperature requirements as your existing fish. Guppies prefer a pH between 7.0-8.0 and a water temperature of 72-82°F.

      Also, it’s essential to keep in mind that the number of fish you can keep in an aquarium is not only determined by the tank size but also by the fish’s bioload and the tank’s filtration system. It’s always better to have fewer fish in a larger tank than overstocking it.

      It’s also important to research the specific care requirements of any new fish you’re considering adding to your tank and observe them for any signs of stress or illness.

  3. Hi Jason, I recently Purchased a 40 Gallon Tank, It will be heavily planted, I am looking to house
    Congo tetras
    Cherry barbs
    Emerald Corys

  4. Hi Jason, we have a 40 gallon tank setup at our house and we want to add some new fish to the fish that we have already.

    Currently in the tank:
    1 Opaline Gourami (main fish had her forever)
    1 female swordtail (needs friends, got from a friend of ours)
    1 bristlenose pleco
    2 khuli loaches

    What could we add to make it more of a community, the fish inside seem lonely

    Thank You!

    • Hi Hunter,

      You can add:

      – A group of 4-6 Neon Tetras
      – A group of 3-5 Zebra Danios
      – 2-3 Pearl Gouramis
      – 3-5 Corydoras Catfish
      – 2-3 Guppies

      Make sure to research the compatibility of each species and the potential size they can reach to ensure they have enough space in the tank.


  5. I have 20 black skirt tetras, 6 Mackey mouse patties, 4 black mollies, 1 sailfin pleco, 3 clown loach, 5 glo danios, 3 yoyo loaches and 1 rainbow shark.
    All are thriving in my 75 gallon and I have fish at all levels and plenty of cleaner fish. But I know that my clown loaches and pleco will get much larger so how many will I be able to keep when they start to fill out?

    • Hi Tom,

      It’s difficult to say exactly how many fish you will be able to keep in the future, as it depends on several factors such as water quality, filtration, feeding habits, and overall health of the fish. However, as a general rule of thumb, a 75-gallon aquarium can comfortably house about one inch of fish per gallon of water.

      The sailfin pleco can grow up to 18 inches, so it will take up a significant amount of space in your tank. The clown loach can grow up to 12 inches, and the rainbow shark up to 8 inches. Keeping in mind the size they will reach, it’s possible that you may need to consider rehoming some of them in the future to ensure they have enough space to swim and live comfortably.


  6. Hello I have a Breeder Tank I think roughly 47 gallon I have:
    1 pictus catfish
    2 clown pleco
    6 tiger barbs
    3 golden clown loach
    6 serpae tetra
    3 Paleatus corys
    And some ghost shrimp

    Everything will roughly grow between 2-4 inches and the pictus maybe will get up to 4-5

    Is that to much for the tank

    • Hi Kyle,

      Based on the size of your tank, the fish you have listed could potentially overstock your tank. A 47-gallon tank is on the small side for these species of fish, especially considering their adult sizes and the number of fish you have.

      As a general guideline, you should aim to have no more than one inch of fish per gallon of water in your tank. With the fish you listed, you have a total length of approximately 42-44 inches, which is well over what your 47-gallon tank can accommodate.

      Overstocking a tank can lead to poor water quality, stress, and disease among your fish. I would recommend either reducing the number of fish in your tank, or upgrading to a larger tank to ensure that your fish have enough space and a healthy environment.


  7. Hi! I have a 40 gal tank and 3 goldfish that we have had for years. We have been wanting to get them some friends and recently added some new species and just wanted to make sure that they aren’t over crowded now. Here are the new additions:
    Pictish catfish
    Dalmatian Molly
    Pink glofish
    2 snails

    • Hello! It’s great to hear you’re interested in expanding your aquarium and providing your fish with new friends. However, it’s essential to ensure you’re not overcrowding your tank, as this can cause stress, disease, and even death for your fish.

      Based on your provided information, it’s difficult to give a definite answer on whether or not your tank is overcrowded. In general, a good rule of thumb is to have no more than one inch of fish per gallon of water, but this is not a perfect guideline, and there are many factors to consider, such as the size of the fish, their activity level, and their waste production.

      With your current setup, your 3 goldfish will likely reach a size of 6-8 inches each, which means they’ll take up a significant amount of space in your 40-gallon tank. The plecostomus and pictus catfish are also large fish that can reach up to a foot in length and produce a lot of waste. The Dalmatian Molly and Pink Glofish are smaller but add to the tank’s bioload.

      Overall, your tank may be overstocked with adding these new fish, especially considering the size and activity level of some of the species you’ve added. It’s essential to regularly test your water quality and monitor the behavior and health of your fish to ensure that they’re not stressed or overcrowded. If you notice any issues, removing some of the fish or upgrading to a larger tank may be necessary.


  8. Hey! I wanted to do the first setup. 5 Neon Tetra, 5 Guppies, 5 Zebra Danio, 1 Dwarf Gourami, and 1 bristlenose pleco. im just starting out so is this a good idea? if so, can you help me out with what else to put into the tank? as in which plants, lighting ideas, etc..

    • Hi Juan,

      It’s great to hear that you are interested in setting up your first aquarium! The combination of fish you have mentioned can work well in a 40-gallon tank, as long as you ensure that the water parameters are suitable for all the species you plan to keep.

      When it comes to selecting plants for your tank, you should choose species that are compatible with the lighting you plan to use. If you are planning to use low to moderate lighting, you can consider species like Java fern, Anubias, and Amazon sword. For higher lighting, you can consider species like Cabomba, Ludwigia, and Rotala.

      In addition to plants, you can also consider adding some decorations to your tank, such as driftwood or rocks. These can provide hiding spots for your fish and add visual interest to your tank.

      When setting up your tank, it’s important to cycle it properly before adding any fish. This involves establishing beneficial bacteria in the tank to help break down waste produced by your fish. You can cycle your tank by adding a source of ammonia, like fish food or pure ammonia, and testing the water parameters regularly until the cycle is complete.

      Overall, your proposed setup of 5 Neon Tetra, 5 Guppies, 5 Zebra Danio, 1 Dwarf Gourami, and 1 Bristlenose Pleco can work well in a 40-gallon tank. Just make sure to monitor the water parameters regularly, provide suitable lighting and plants, and cycle your tank properly before adding any fish.

      Good luck with your aquarium setup!

  9. I’m thinking of starting a freshwater 40 gallon breeder tank with the following fish:
    (3) Peacock Gideon
    (6) Cardinal Tetras
    (3) Cory Catfish
    (2) American flagfish (maybe)
    Nerite snails and shrimp
    I’m also interested in celestial pearl danios and the bristle nose pleco but I don’t think I can fit all that. This will be my first tank since I was a kid so I’m open to suggestions


    • Hello Stevie,

      Thanks for reaching out! It sounds like you’re making great plans for your new freshwater 40-gallon breeder tank.

      Your list of fish species seems generally well-suited for a tank of this size, but there are a few considerations that you might want to take into account.

      – Peacock Gudgeon: These are great choices for a community tank as they are peaceful and quite colorful.

      – Cardinal Tetras: They are a great addition, very peaceful and beautiful, and will thrive in a small school.

      – Cory Catfish: These little scavengers are very social and prefer to be in groups, so you might want to consider adding a few more, maybe bringing the total up to 5 if space allows.

      – American Flagfish: While they are generally peaceful, some individuals can be somewhat aggressive or territorial, especially in a smaller tank. If you’re planning on adding these, make sure you have plenty of plants or decorations to serve as hiding spots.

      – Nerite snails and shrimp: These are excellent for controlling algae and providing some extra interest at the lower levels of the tank.

      As for the celestial pearl danios and the bristle nose pleco, both species can be accommodated in a 40-gallon tank, but as you mentioned, you might be stretching the stocking limits.

      The celestial pearl danios are small and don’t produce a lot of waste, so you could possibly fit in a small group of them (around 5) without overstocking the tank. However, bristle nose plecos can get quite large (up to 5 inches or more) and are notable waste producers, which might push your tank’s capacity to its limit.

      If you decide to add more species, you’ll want to pay close attention to your water quality to ensure it stays healthy for your fish. Frequent water changes and monitoring the water parameters are essential.

      Finally, be sure to introduce your new fish slowly, over a period of weeks or even months. This will give your existing fish time to adjust and will also help ensure that your filtration system can handle the additional waste load.

      Happy Fishkeeping!

  10. Hey there, I currently have a 40 gallon tank and I’ve got two African cichlids( one might be the blue zebra), two convict cichlids, 2 blood parrot cichlids, a pleco, and a pictus catfish. They seem to live together fine although occasionally living up to the cichlid name by giving chase and spots of aggression. The biggest fish is the blue African cichlid. At least I was told he’s an African but then I heard about a different kind called a blue zebra something or other. Any advice you could give me with these guys and maybe a reassurance that it’s not too crowded? I’ve been told numerous times there’s room for more but I don’t think it’s a good idea currently. Any tips or advice is appreciated.

    • Hi Josh,

      Thanks for reaching out with your query about your 40-gallon tank setup. Here’s some insight based on the mix of fish you’ve described:

      African Cichlids and Convict Cichlids: African cichlids, including the Blue Zebra (also known as Metriaclima callainos or Malawi Blue Zebra), tend to require different water conditions than the Convict cichlids. African cichlids thrive in harder, more alkaline water while Convicts are more tolerant of neutral to slightly acidic conditions. It’s essential to ensure that water parameters are balanced to cater to the needs of both.

      Blood Parrot Cichlids: These are hybrid fish, and while they can be less aggressive than some other cichlids, their oddly-shaped mouths make it difficult for them to defend themselves or compete for food. Keep an eye out to ensure they’re feeding well.

      Pleco: Depending on the type of pleco you have, it might outgrow a 40-gallon tank. Some plecos can reach over a foot in length. Make sure it has hiding spots and driftwood to graze on.

      Pictus Catfish: Pictus catfish are active swimmers and can sometimes be fin-nippers. They do better in groups, but with your current stocking, I’d be cautious about adding more fish.

      Stocking Level: A 40-gallon tank can quickly become overcrowded, especially with the mix of fish you’ve chosen, many of which are territorial. While they may seem fine now, as they grow, they’ll need more space, both for swimming and to establish territories.

      Aggression: Cichlids are known for their aggression and territorial behavior, especially during breeding. Make sure you have plenty of hiding spots, rocks, and caves in the tank. This will allow weaker or bullied fish to find refuge.

      Given your current stock, I’d advise against adding more fish. Instead, focus on ensuring optimal water quality, providing enriching environments with hiding spots, and closely monitoring their behavior. If you observe prolonged aggressive behavior, consider rehoming or separating the culprits to prevent injuries and stress to other tank inhabitants.

      I hope this helps, and best of luck with your aquarium!

      Always happy to assist fellow aquarists.

      Best regards,


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