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

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by Jason Matthews


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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

2 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)


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