Why Is My Cardinal Tetra Aggressive Towards Other Cardinal Tetras?

It can be challenging to determine why a cardinal tetra is aggressive towards other cardinal tetras. These fish are usually very peaceful, and they live in schools together peacefully most of the day. However, there may be many reasons why your pet cardinal tetra has become aggressive.

Some of the reasons why Cardinal tetra is aggressive towards other Cardinal tetras are:

  • First, the tank size is too small.
  • Second, there are not enough hiding places in the aquarium for the fish.
  • Third, the water parameters are incorrect.
  • Fourth, one or more of the fish is sick and stressed out.
  • Finally, there is a lack of food.

Below we will explore some possibilities for why this may happen and how to manage this.

Are Cardinal Tetra Aggressive Towards Other Cardinal Tetras?

The cardinal tetra (Paracheirodon axelrodi) is a peaceful, schooling fish popular in the home aquarium.

However, sometimes cardinal tetras can become aggressive towards other cardinal tetras. It can be particularly problematic if you have a school of cardinal tetras in your tank. So, why do cardinal tetras become aggressive towards one another?

Why Is Your Cardinal Tetra Aggressive Towards Other Cardinal Tetras?

There are a few possible reasons. Such as:

Inadequate Tank Size

A common problem seen in the aquarium hobby is overstocking, which can cause aggression. For example, overcrowding a school of cardinal tetras would be disastrous as they are famous for attacking other fish and even cannibalizing each other. In general, you should only keep cardinal tetras with others of their kind (cardinal tetra fish) in a tank of at least 30 gallons.

It may be the reason for your cardinal tetra’s aggression, as it feels threatened by the other fish in its environment.

Poor Water Quality

Another potential cause of aggressive behavior in cardinal tetras can be poor water quality. Cardinal tetra is freshwater fish and cannot tolerate changes in water chemistry, such as high levels of ammonia or nitrite. Therefore, if the water quality in your aquarium is not up to par, it can cause stress and lead to aggressive behavior.

Competing for Food

A potential reason for cardinal tetra aggression can be competition for food. When there is much fish in the tank, it can lead to tension and fighting over resources. It can be especially true if only a few pieces of food are available.

Lack Of Hiding Place

In the wild, cardinal tetras live in dense vegetation to hide from predators. However, in an aquarium, if there are not enough places for them to hide, they may become stressed and more aggressive as a way of defending themselves.

Incorrect Water Parameter

If you have recently changed the water parameters in your aquarium, it can also lead to aggression in cardinal tetras. For example, if you have raised the pH level or increased the hardness of the water, it can make the fish uncomfortable and cause them to become aggressive.

Illnesses Or Diseases

Cardinal tetras are prone to various diseases and illnesses, and one of the symptoms can be aggressive behavior. If you have noticed that your cardinal tetra has become more aggressive than usual, it is crucial to take a closer look at its health and treat any illnesses as soon as possible.

Incompetent Tankmates

If you have chosen inappropriate tankmates for your cardinal tetras, it can also lead to aggression. For example, suppose you have added a fish that is much larger than the cardinal tetras. In that case, they may become intimidated and start attacking other fish in an attempt to defend themselves.

What To Do If My Cardinal Tetra Is Aggressive Towards Other Cardinal Tetras?

Aggression in Cardinal tetras is unusual. While this can be an issue in some cases, there are ways to fix the problem. Such as:

Introduce Your Cardinal Tetra In Bigger Tank

Many people mistake putting their tetras in too small a tank. Unfortunately, this can cause them to become aggressive as they feel threatened by other fish and want to protect themselves from potential threats. So, if you notice your Cardinal Tetra becoming defensive, it’s time for an upgrade.

Maintain An Optimum Water Parameters

The water in your tank is equally important. Ensure that the parameters are correct to avoid aggression issues between Cardinal Tetras and other fish. It means keeping up with pH, ammonia, nitrite levels, as well as any medication you put into the water for treatment of illness or parasites.

Pick Fish That Get Along Well Together

Sometimes you can avoid aggressive behavior by choosing fish that get along well together. There are many different types of Tetras and other small fish to choose from, so take your time picking the right ones for your tank.

Add Some Decorations

Adding decorations can also help to diffuse any aggression issues between Cardinal Tetras. It could be something as simple as a few plants or driftwood. It gives the fish something to focus on other than each other and can help to calm them down.

Introduce New Fish Slowly

If you decide to add new fish to your tank, do so slowly. It will help the fresh fish adjust to the water conditions and allow the dominant fish to become familiar with them. However, adding too many new fish at once can cause severe problems.

Give Enough Food And Space

Finally, giving your Cardinal Tetras enough food and space is essential. If they are constantly feeling hungry or crowded, you will see more aggression in them than usual. Feed them a good diet of quality food and give them enough room to swim around.

If all of these methods fail, you may have to consider separating your Cardinal Tetras into different tanks. It is not always ideal, but it may be necessary if the aggression continues to cause problems in your tank.

What Are The Fish That Get Along With Cardinal Tetra?

Some fish that get along with cardinal tetra are other small, peaceful schooling fish. They are:

  • Neon Tetra
  • Harlequin Rasbora
  • Celestial Pearl Danios
  • Betta
  • Guppies
  • Mollies
  • Platys
  • Swordtails.

They are a good choice for those with multiple cardinal tetras that they want to keep together. They will school with your fish and help create a fun environment!

What Are The Fish That Do Not Get Along With Cardinal Tetra?

Cardinal tetras are usually the ones that have problems among community fish. You can add many different types of fish with cardinal tetra. But if you want to keep them together, it is best to choose species accordingly.

Some fishes that cannot get along with Cardinal tetra are:

  • Plecostomus Catfish
  • Siamese Algae Eater
  • Silver Dollar Fish
  • Tetras (other than Cardinal tetra)

If you have any of these fish in your aquarium, it is best to remove them, or else you will have a lot of injured and dead cardinal tetras.


What Is The Best Water Temperature For Cardinal Tetras?

The best water temperature for cardinal tetra is between 70 and 78 degrees Fahrenheit. If you keep the fish with a heater, you can maintain this temperature by adjusting it accordingly.

What Is The Best Water Condition For Cardinal Tetra?

Cardinal tetra will do well in most aquariums as long as there is good water circulation and the pH level is six and eight. It would be best if you also kept nitrates below 20 ppm and ammonia levels at 0ppm for your tetras’ health.

How Many Cardinal Tetra Can I Put In My Tank?

You can put anywhere from five to ten cardinal tetra in a tank that is twenty gallons or more significant. You can even put more in a more giant aquarium, but keeping them in small groups instead of large ones is best.

What Is The Best Tank Mate For Cardinal Tetra?

Cardinal tetras are peaceful fish that get along well with other non-aggressive community fish. Some good tank mates include clown loaches, cories, white cloud mountain minnows, and zebra danios.

What Is The Best Food For Cardinal Tetra?

Cardinal tetras are omnivores that eat both animal and plant matter in the wild. In an aquarium, you can feed your cardinal tetra most types of food, including flakes, freeze-dried bloodworms or tubifex, and freeze-dried brine shrimp.


In any case, it’s essential to keep an eye on your fish and address any aggression issues as soon as possible to avoid harming the fish or causing a messy tank. You can often resolve the aggression of your pet with some simple changes. So, don’t hesitate to try different tactics.

This post comes to an end now. However, for more information about Cardinal Tetra, visit other posts on our website.

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