How Many Fish Should You Have In A 40 Gallon Tank

There are thousands of species of fish that you can keep in your home aquarium. They will come in every size that you can imagine. Some fish do not mind tank mates, while other fish are aggressive and should be kept alone.

You also have some species of fish who prefer to be in schools and some that are content with one or 2 cohabitants. Most fish you keep love to have a few hiding spots in the aquarium. But how do you know how many are safe to keep in the tank?

How many fish can you have in a 40-gallon tank? If the fish size is compatible with the shape and size of the tank, then 25 is a good number to have. You can also go by the rule of 1 gallon of water per 1 inch of fish length. For larger fish, such as goldfish, calculate 2 gallons of water per 1 inch of fish length. It ultimately comes down to the size of the fish that you want in your aquarium.

You do have to take into account that the water volume in your tank is often 10-15% less than the size of the tank. This is due to items that you put in your tank; such as substrate, live plants, rocks, and other decorations. You also can not fill it to the brim with water. So, in reality, your 40-gallon tank will have between 34 and 36 gallons of water in it. This will lower the number of fish that are safe to have. 

I will cover 40-gallon tanks; shapes you can purchase, care, and types of decorations. I will also give you some examples of fish that you can keep in your 40-gallon tank and how many are safe and healthy. If you have too many fish in your tank it is not healthy and they are not going to be happy.

About 40-Gallon Fish Tanks

The 40-gallon aquarium is the most popular size of the aquarium. Aquarists and beginners love the size because of how versatile it is. You can keep a variety of beautifully colored fish in a 40-gallon tank. It is the perfect starter tank and offers ample room to put many hiding spots for your fish. With a 40-gallon tank, you have the option of having a freshwater or a marine (saltwater) set-up.

Fish Tank Shapes

There is, of course, the most popular rectangular tank. But you can also purchase tanks that are in the following shapes/styles:

  • Hexagon
  • Pentagon
  • Corner bow front
  • Half cylinder
  • Bull nose
  • Bow front

Most of these tanks will be made from acrylic instead of glass because the acrylic is easier to manipulate and shape. These types of tanks are also a little harder to clean, because of all of the extra crevices, but they are aesthetically pleasing to the eye.

Adding a different element to the room that you keep it in. You are also limited with the tank shapes, on how you can decorate. They can be creative and unique. 

Aquarium Equipment

There are certain items that are must-haves no matter what size aquarium you possess. They are essential in keeping your fish healthy and happy. The following items are the most important/essential items:

  • Filter: these create water flow, help keep the water aerated, and helps keep the tank clean.
  • Heater: this is very important because fish can only survive in a specific temperature range. Normally the temperature is going to have to be set between 74 degrees and 78 degrees. Most heaters will come with a thermostat that will shut the heater off when it reaches the designated temperature. Read the box to make sure. 
  • Lights: these mimic the daytime cycle that fish are used to when they are out in nature. It is best to also have a timer so that you can set your lights to go off/on in 12-hour intervals. They also aid in the growth of your live plants, your fish, and the good bacteria in the tank.
  • Accessories: gravel cleaners, sponges, automatic feeder, aquarium stand, and other odds and ends. 

Cleaning Your Fish Tank

Any aquarium is fairly easy to clean, as long as you have higher quality equipment for your tank. As long as your filter is always working at full capacity you can get away with a partial water change every 7-10 days (or once a week). 

Your partial water change will consist of the following; cleaning your filter (changing the media if needed), vacuuming or siphoning your substrate, and changing 25% of your water. This process gets rid of food waste, fish waste, and other debris that could be in the tank. It also helps to ensure your fish will stay healthy.

How Many Fish In 40-Gallon Aquarium

How Many Fish Can You Have In 40 Gallon Tank

Depending on the fish size, when they are adults, you can keep a good number of fish in the 40-gallon aquarium. They need to have enough room to swim around without running into each other or any of the decorations.

It is suggested to only have 30% of your tank decorated, giving your fish 70% capacity to swim around and play. You do need to make sure if you are going to have more than 1 species in the tank, that they are compatible. 

Some of the most ideal fish for your aquarium are;

  • Diamond neon tetra
  • Rainbowfish
  • Fantail Guppy
  • Cherry barb
  • Mollies
  • Zebra danio
  • Yellow cichlids

There are many others that are perfect for your tank as well. As long as they are compatible, have enough room to swim around, and have high-quality equipment you can have just about any fish in your tank.

Fish Tank Considerations

There are many fish who love to be kepts in schools, like guppies. If you are going to choose a schooling fish, make sure you have 15-20 together. This makes the fish feel secure, happy, and they will flourish.

You want to have different species that are very close in size. This will prevent the issue of bigger fish eating smaller fish. You do not want to come home from work one day and have all of your little fish gone!

Again, you do not want to put aggressive fish together in a tank. Example: the betta fish. Males are extremely territorial and you can not have 2 males in the same tank under any circumstances.

There are other species of fish out there that are the same way, so make sure you do in-depth research. This will guarantee fights, stress, and unhealthy fish. Peaceful fish who swim around, play, or keep to themselves are recommended.

If you want to do bigger fish, you may only be able to have 2 or 3 in your 40-gallon tank. The number will depend on their size as an adult. Remember the rule; for bigger fish, you want to have 2 gallons of water per 1-inch length of fish. So, if you have a 12-inch fish, as an adult, 2 in a 40-gallon tank will be pushing your luck.

Aquarium Stocking Idea

When aquarists have a 40-gallon tank, most will stock it with colorful fish that compliment each other. Then you can add live plants and decorations to make a beautiful fish community. Rocks, substrate, live plants, a ship, or other fish tank decorations can make the home perfect for your swimming friends.

On set-up that repeatedly came up in all of my research:

  • 5 neon tetra
  • 5 zebrafish (zebra danio)
  • 5 guppies
  • 1 dwarf gourami
  • 1 bristle nose pleco

There are many other variations that you can do. Just goldfish, guppies, tetras, zebra danios, and so much more. It will take quite a bit of research on your part to make sure that the combination you want is going to be compatible together. You could also opt to spend a couple of hours in a pet store talking to the employees about which fish are the best tank mates.

Not only do your fish have to be compatible, but they have to enjoy the same temperature, pH levels, food, and water current. It will not be a fast or easy task to get the perfect combination of tank mates. But once all the research is done and you have it all together, it will be a focal point of the room.


Depending on fish size, you can have a good number of fish in your 40-gallon tank. The suggested number is 25. You can also use the, 1-gallon of water per 1 inch of the fish rule. This would call for more than 25 fish in your 40-gallon tank. I, personally, wouldn’t have more than 25 or 30 fish in a 40-gallon tank. Err on the side of caution to have happy and healthy fish.  2 gallons of water per 1 inch of fish, reduce the number quite a bit. 

When calculating how many fish you can comfortably have, remember that your tank will have 10-15% less water volume. So you will actually have 34-36 gallons of water in your 40-gallon tank. That is still quite a large number of fish, depending on their size. 

One final reminder: the species you choose to put in the aquarium together MUST be compatible to ensure a stress-free community. 


I'm passionate about fish pets and love sharing everything I learn about them.

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