How Many Fish Can You Keep In A 40 Gallon Tank?

A 40 gallon aquarium is a great size for a beginner, but it’s not large enough that you can keep everything in it. A key part of having a successful tank is knowing how many fish you can safely keep in it. 

A 40 gallon breeder tank can hold 1-2 medium sized fish or 20-25 small fish like neon tetras. For saltwater aquariums, a 40 gallon tank can hold 5-6 small fish in addition to corals and other inverts. The common rule of thumb is 1 inch per gallon, but this only works for fish smaller than 3″.

Figuring out how many fish you can keep in your 40 gallon tank can take some time as there are multiple factors to think about. This article will go through these factors and how they will affect your choice of fish for your tank. 

School of Harlequin Rasbora | Source: Deposit Photos

How Many Fish Can A 40 Gallon Tank Hold?

The amount of fish that you can keep in a 40 gallon tank will vary greatly depending on multiple factors. These Factors include:

  • The size that your chosen fish will be when they are fully grown
  • The bioload caused by the fish
  • The filtration of your tank
  • The aggression levels of each of the fish you are stocking your tank with

The best thing to do is first decide on the type of fish you want to keep and then do some research on what living conditions they require, how big they get, and their aggression level. Then you can determine what other fish you can add to your tank with them. 

Considerations Before You Stock Your Tank

There are countless combinations of fish that you can stock in a 40 gallon fish tank, but before you stock your 40 gallon tank with fish, there are some things that you need to go through and consider so you can keep all your fish healthy and live.

Cardinia Red Bee Shrimp | Source: Deposit Photos

These few factors we will go through below will affect the number of fish you can stock your tank with. So, let us look at them so you can stock your tank correctly. 

Waste Load From The Fish

Before you stock your 40 gallon fish tank, you need to consider the waste produced and how you are going to clean it all. With aquariums, there is a nitrogen cycle; this refers to when a fish eats, they then produce waste. This waste is then broken down by beneficial bacteria and living plants that are kept in the aquarium. 

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However, if there are way too many fish in the tank compared to the living plants and beneficial bacteria, the waste will begin to build up. When this happens, the quality of the water will go down, leading to your fish getting sick or even dying. 

This is an excellent reason not to overcrowd your fish tank as you will spend more money cleaning the tank and replacing dead fish than doing anything else.  

You need to ensure you have a few things in your tank to help minimize waste as much as possible, no matter how many fish you have in the tank. You need to make sure that you have a good filtration system installed to help clean the water and reduce toxins. 

Then you need to get an adequate number of living aquarium plants that will help remove nitrogen from the water as this is toxic to fish. As an aquarium owner, it is recommended that you own an aquarium water test kit, like this one.

The test kit will allow you to check the levels of nitrate and ammonia in the water. Knowing these levels will help you determine if you need to replace the water in your tank or not so you can keep your fish healthy.

Swimming Space Required For Each Fish

You need to keep in mind the space requirements of each fish you want to get before you stock your tank. There have been some recommendations in the past to beginners that you should keep one inch of fish per every one gallon of water in your tank. 

However, this rule only applies to small community fish, about 1-3 inches (2-7 cm) in length. A good example is that ten 1 inch tetras put together do not have the same body volume as a 10 inch Oscar fish. 

TIGER OSCAR FISH astronotus ocellatus, CICHLID OF SOUTH AMERICA | Source: Deposit Photos

So, the bigger the fish you want to keep in your tank, the more swimming space they will require, and this is a vital fact to consider as it will impact the health of your fish. Remember when you buy fish for your tank to use their fully grown size to determine how many you can keep in the tank. 

Aggression Levels Of The Fish

The last thing to keep in mind before stocking your fish tank is the aggression level of the fish you want to get. Some fish are more aggressive and dominant than others, which can cause issues in your tank with fish fighting and even killing each other.

For example, the best thing to do with African cichlids is to add as many of them as you can into your tank. This decreases their swimming area and lets no single fish have enough room to establish dominance and defend its territory. 

lake Malawi cichlids | Source: Deposit Photos

Or Betta fish that usually stay at the top of the aquarium may get aggressive towards any fish that swims near the top of the tank. So, you need to choose fish that will swim in the middle of the tank if you want to pair them with others.

You can also add some more plants and other decorations into your tank to break up the aggressive fish’s line of sight, allowing the weaker fish to hide and escape if they need to.

Stocking Ideas For A 40 Gallon Fish Tank

As a 40 gallon fish tank is quite large, you can keep a good variety of fish in it. This section has a few ideas of what fish and how many of each fish you can keep in your 40 gallon tank. These fish are: 

  • Guppies. You can keep about 20-30 guppies in a 40 gallon tank, keeping them to a group of at least 6
  • Cory Catfish. These fish will help clean your tank of algae, and you can keep up to 10 full sized corydoras in a 40 gallon tank. (You can keep a lot more if using one of the dwarf varieties.)
  • Mollies. These fish are stunning and energetic. You can keep about 15-20 of them in a 40 gallon tank
  • Three Spot Gourami. You can keep up to 10 of them in a 40 gallon aquarium. 
Male Guppies | Source: Deposit Photos


A 40 gallon tank is large tank size, so this does offer you some great fish options to keep in it. You need to research your fish properly to determine how big they will get when fully grown and how aggressive they will be, as these two main factors will determine how many fish you can keep in your aquarium.