How To Pick Fish For A 40 Gallon Aquarium


So you’ve decided on a 40-gallon tank – Fantastic! Forty gallons is a fantastic choice, whether this is your first tank or fiftieth. This size of aquarium gives you plenty of options for decorations and types of fish. When first starting out as an aquarist, choosing a larger tank can be intimidating, not to mention costly if you make a mistake.

On the other hand, small tanks need tons of regular maintenance that may be beyond the level of work you want to perform. The 40-gallon tank is a perfect middle option between the quantity of space and the extent of upkeep.

Initial Considerations

Now that you’ve chosen your tank size, there are a few other considerations to ponder before you start building it into an aquarium. We’ll touch on what kind of set-ups you can make later in the article, but first, you need to consider your overall purpose in building this tank.

You’ll then need to decide how much work you want to do, what kind of water you will need, decor options, and compatibility of the fish you’d like to keep.

What do you want to accomplish with this tank?

The most common reason for wanting to build an aquarium is that they’re beautiful! But is that the only reason you have for all the time, work, and funds that go into a tank? If so, that’s completely fine! Like every other reason for building a tank, as long as you know what you want, you can start making decisions on different criteria.

For example, suppose you’re looking at this tank’s size specifically because it is a popular choice for breeding. In that case, you will have different equipment, water parameters, and decor options than if you want the tank to house an aggressive Oscar.

How much work do you want to do?


No matter what kind of freshwater set-up you decide on, every one of them will require some sort of work to keep it fresh and running correctly. But, some types require more work than others. Every living thing you put in your tank has what is called a ‘bioload.’ The bioload refers to the amount of waste that the plant or animal produces.

For fish, this means mucus, poop, and food that the fish miss at dinner time and settles in the tank. Different fish species have different bioloads, and different tank set-ups will handle these bioloads in different ways. It’s essential for many reasons to research the fish you want to keep. Not least of these is to know how often you will need to do tank cleans and water changes.


Just like people, not all fish get along with each other! When deciding what kind of build you want to make out of your 40-gallon tank, you need to know if the fish you have in mind are compatible. If you’re going to keep more than one species in your tank, it is easiest to choose all peaceful community fish with similar activity levels.

You certainly have other choices, but no matter which you go with, be sure to do your research to ensure they have suitable neighbors.


Most fish species will feel comfiest in an environment that is similar to their homes in nature. Even if your fish were tank bred, most fish species would still show their best coloration and activity levels if you create an environment like their native homes.

If your fish originates from fast-moving rivers with sandy beds, having a high water flow and sand substrate will make them feel right at home.

Water parameters

Different species of fish will also have different needs from their water quality. When choosing multiple species, you need to be sure they all have similar requirements for pH levels, water temperature, and hardness.

Any reputable fish seller, whether in person or online, will have stated the optimum water parameters, so you should easily be able to tell if all your fish will be happy in the same type of water.

Introducing fish to the tank


When you bring home new friends from the fish store, you have to follow specific steps when introducing them to their new environment. First, place the fish in the aquarium water while still sealed in the bag. Allow 10-15 minutes for the water temperature to equalize. Next, make a cut at the top of the bag and slowly add small amounts of tank water through the opening.

This is to make sure the fish isn’t shocked by the difference in water parameters. After another 10-15 minutes of slowly adding water, you can pour the bag water into another container under a net to catch the fish. Then you can move the fish into their new home. 

Depending on the kind of fish you’re keeping, and how many of each you have, you will need to add fish to the tank in stages. Pick your most timid fish to add first, and follow them with more aggressive or active species. This will allow your shyer fish to check out the tank without being disturbed. It’s also good to add just a few fish at a time.

If you’re planning on keeping several species, or a large group of a single species, it can be a fun treat for yourself (or your kids, if they’re involved in the process) to go to the fish store over multiple days to pick out new pets. 

Tank Theme

The 40-gallon aquarium gives you tons of options to choose from when it comes to stocking. It is a perfect option whether you would like to keep a single fish, a group of the same species, or several different species.

Whichever set-up you choose, remember to stick to the rule of thumb to house no more than one inch of fish per gallon of water your aquarium holds. Forty inches of fish is a lot to choose from, so we’ve listed below some of the most popular themes for 40-gallon freshwater aquariums.

Species Only

If you really want to learn the most about a particular species of fish, their habits, and the kind of care they need, a species only tank may be for you! There are tons of freshwater fish options that would love a 40-gallon tank and make absolutely stunning additions to your home.

Cardinal tetras are one option that is popular amongst intermediate and experienced aquarists. Their shoaling behavior and bright red and blue stripes are fascinating to watch. These fish only grow to a maximum of 2 inches, so you will easily be able to keep a group of 20 in a species only tank. Other great options for a species only aquarium are:

  • Diamond Tetras
  • Dwarf Neon Rainbowfish
  • Dwarf Cichlids
  • Full-Size and Dwarf Gouramis
  • Glass Catfish
  • Tiger Barbs

Single Fish


While you may think that more fish makes a more exciting tank, a single fish tank can also be remarkably lovely and interesting to watch. Types of fish that do well in a single fish tank are those that are often aggressive. They may bully other fish to defend their territory or even eat smaller fish to ensure they are the top dog-er, fish in the tank.

Interestingly, these large, aggressive fish often form bonds with the humans that feed them and interact with them throughout the day. These types of fish usually need a high protein diet with lots of meaty live and frozen foods, so be sure you’re ready to deal with their voracious appetites! Some excellent choices for a single fish 40-gallon tank are:

  • Jewel Cichlid
  • Flowerhorn Cichlid
  • Green Terror Cichlid
  • Paradise Fish

Peaceful Community Tank

The peaceful community tank is one of the most popular setups for aquarists of all levels and tanks of many sizes. However, the 40-gallon tank is one of the most popular size choices for this theme, as it offers enough space for a variety of species without the large space commitment of a larger tank.

The peaceful community tank will provide lots of color and activity and are often very relaxing to watch. There are so many options for peaceful community fish that it can be hard to choose! A popular strategy to limit the number of choices is to look at between one and tree species for different swim levels of your aquarium. 

The tank ‘swim levels’ refer to where your chosen fish like to spend the most time. The levels are top, middle, and bottom. Excellent examples of bottom swim level fish for the community aquarium are cory cats, dwarf shrimp, kuhli loaches, and bristlenose plecos. Many tetras, dwarf gouramis, rainbowfish, and barbs make great additions as middle-level swimmers.

Pencilfish, danios, hatchetfish, and halfbeaks give lots of color and activity to the top of your tank.

In addition to these fish groups, many aquarists choose to have one to three of what are called ‘centerpiece fish.’ A centerpiece fish is just what it sounds like, one (or a few) very colorful fish that are complemented by all the other fish and decorations. These are the show-stopper fish that will make your guests’ eyes go wide and say, “WOW!”.

Popular options for centerpiece fish in a 40-gallon aquarium are dwarf cichlids, dwarf rams, and Dwarf Gouramis.

Semi-Aggressive Tank

If the quiet community tank isn’t something you think you would enjoy, a semi-aggressive tank may be right for you. Semi-aggressive fish often have a higher activity level than peaceful fish, making for a more exciting viewing experience. With semi-aggressive fish, you need to make sure that your tank has enough space for each individual to have their own territory.

To break up the aggression, be sure to add lots of dense plant cover, decorations with hiding spaces, and large rocks, driftwood, and crevices for fish to escape and hide if they are being bullied. 

Semi-aggressive fish often grow larger than their peaceful counterparts, so make sure to know how big your fish will be once they are fully grown. This will ensure that when your fish reach adulthood in several months, they will have enough space, and you won’t have overcrowding issues. Popular choices for semi-aggressive fish in a 40-gallon tank are:

  • Angelfish
  • Semi-aggressive loaches (yoyo, skunk, bumblebee, etc.)
  • Full-size gouramis (blue, pearl, etc.)
  • Sharks

Some fish that are considered peaceful, such as multiple species of danio, some barbs, and some tetras, are fast-moving enough to be able to hold their own in a semi-aggressive tank. I successfully kept a school of pearl danios in my 40-gallon semi-aggressive tank. Just be sure to check out the fish’s activity level in the store when considering any non-aggressive species for this tank theme.

Betta Sorority


A betta sorority tank is a mix between a species only tank and a semi-aggressive tank, but we thought it deserved a section all on its own. While some aquarists have successfully kept a betta sorority in as small as a 20-gallon tank, a 40-gallon will be a safer option. Betta fish are usually kept alone in aquariums due to their aggressive nature.

However, female bettas are less hostile than their male counterparts and can be housed together under the right circumstances. As with other aggressive and semi-aggressive fish, make sure there are plenty of territory markers and hiding spots to make sure no one fish is singled out to be bullied too often. 

Female bettas reach about two and a quarter inches in length when fully grown. You should still expect to keep only around 10-15 individuals in a betta sorority because of their dominant nature. Each betta will have her own personality and tolerate others’ company differently, so make sure you introduce individuals to the tank in stages and keep a close eye out for any fighting.

Although they often lack the dramatic tails of males, female bettas still come in a wide variety of colors and body shapes. You can choose all different types to have multiple colors and shapes, or you can pick all one species or those that look similar to each other to create a more unified look.


A 40-gallon tank is a fantastic option for beginners and experts alike. It offers enough space for multiple stocking options but is manageable enough to fit most homes. It is also a great middle option between the amount of space it offers with the amount of upkeep you will have to perform.

You can choose to stock your tank with a single aggressive fish, a multitude of colorful, peaceful fish, and everywhere in between. No matter which tank theme you choose, a 40-gallon freshwater aquarium will be sure to be a lovely and exciting addition to your home.

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