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How Many Fish in A 40 Gallon Tank

A 40-gallon tank is desired by every aquarist, but somehow it comes with the great responsibility of caring. When you have a 40-gallon fish tank, the first thing occurs in your mind must be the number of fish that you can keep in it. The general rule for stocking fish is one inch per gallon, but in a tank like this, you can keep at least 20-30 fish.


All about the 40-gallon fish tank

A 40-gallon tank is a perfect aquarium for both freshwater and marine aquarium fish. We got a perfect space for decorating with more aquatic plants and lights that’ll give a unique look to the tank.

A perfect 40-gallon tank weighs 60lbs and has dimensions of 36″x13″x20″. It is also called a breeder tank and has a slight difference dimension of 36″x18″x16″.


Why are 40-gallon aquarium called a breeder tank?

The specific dimension of 36″x18″x16″, which is designed for aquarists to easily watch out for fish due to its lower profile. In this tank, usually, the bottom base area is square-shaped. This structure helps us to easy caring for fish and we also adjust the positions and layouts of substrates and other things inside the tank.

We can easily get it through online websites and amazon. Apart from such advantages, there are some limitations too.


What are the advantages and disadvantages of a 40-gallon fish tank?


  • It gives a larger viewing area which helps us to enjoy or stock fish behavior even from a distance.
  • It will be easy maintenance due to its big size. We can easily clean up the tanks and even in large tanks the changing of water is reduced.
  • It gives us the choice of a variety of fish from different environments like marine or freshwater tanks.



  • Initially, we have to invest more to set up the tank in our desired way.
  • A large tank means that the equipment like heaters, water pumps, lights, etc., should have more power to function for the entire tank.
  • Need enough space for about 40*20 inches in your room or office, and should be away from windows, or any heat-emitting machinery like AC, Oven, Refrigerator, and so on.


What are the factors should we consider before stocking our tank?

There are thousands of combinations of fish for a 40-gallon fish tank, but overcrowding is not a good thing. So, for a healthy tank, we should consider some factor that not only keeps our fish healthy but also maintains a good ecosystem. The factors are as follows:


1. Fish become aggressive

Fish have nature of showing their territorial behavior and establishing their dormancy whether for freely swimming or for females while the mating period. So we should avoid adding aggressive fish with the small or peaceful fish and also add more plants and decorations for other fish to hide.


2. More fish leads to more waste

It is obvious more fish will create more waste in the tank, which increases the toxicity in the tank. We should add tank cleaner (fish) to the tank, and also fit a good filtration system. We have to test the water quality using a water test kit that gives us the signal to change the water of the tank.


3. More swimming space required

We have to research before adding any fish to our list. After attaining their full adult size they need their proper swimming area, so to stop quarreling for space, we have to reduce the number of fish and also avoid adding bigger fish.


What are the 40 gallon fish tank stocking ideas?


To make our aquarium more beautiful then we should add unique and colorful fish to our tank. The types and number of fish that you can keep in a 40-gallon tank are as follows:

  1. Guppy- These are small in size and vary in color. We can add 20-30 guppies to this tank. They are peaceful and social living fish.
  2. Betta- These fish have beautiful fins and colors. They are quite aggressive so we can add 12-15 female betta and 1-2 males.
  3. Cory Catfish- They are good tank cleaners and also very peaceful fish. They live in themselves and also help in controlling algae growth. We can add 10-12 Cory catfish in a 40-gallon tank.
  4. Banggai Cardinal fish- These fish with such unique structure increases the value of the tank. They are also quiet and peaceful in nature and can add 15-20 Cardinal fish to our tank.
  5. Zebra fish- We can add 16-20 zebra fish to our 40-gallon tank. They are naturally social fish but they are found always swimming inside the tank.
  6. Mollies- After Zebra fish, one more energetic fish is available for the tank. These fish always entertains their breeders and we can add 15-20 of them. They always love to swim or play around tall plants.
  7. Goldfish- Goldfish are the most desired fish among all fish. We can place only 2 goldfish in a 40-gallon tank. It widely depends on the type of goldfish.



    A good ecosystem in the aquarium is based on the correct decision we make. Hope this article “How Many Fish In A 40 Gallon Tank” helps you to decide what should be the best for your 40-gallon tank.

    If you have any unanswered questions or want suggestions for the products like a heater, a filtration system, and more to manage your aquarium then please leave your message in the comment section.



    1. What should be the ideal flow rate of the filter in a 40-gallon tank?

    The suitable rate should range between 180-200 gallons per hour.


    2. What should be the capacity of the heater in a 40-gallon fish tank?

    A heater with a 150-watt capacity is ideal for the 40-gallon tank.


    3. Can you breed fish in a 40-gallon tank?

    A 40-gallon tank is perfect to breed between any small or mid-sized fish with a few large species of fish. This tank is perfect for spawning fish.


    4. What is the largest number of fish which should be added to the 40-gallon tank?

    We can properly add up to 30 fish of the same species inside the tank. Perfect examples are guppies and more which are mentioned in the article above.

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