Why Are Aquarium Fish Eating Each Other?

Cannibalism among aquarium fish (i.e., fish-eating each other) is common, but sometimes these do end up gobbling there cohorts. There is no definite answer to why this happens.

However, it does impede your desire for having a well-maintained aquarium in your living space.

Sometimes it’s natural, while sometimes it’s due to fish being opportunistic.

Let’s discuss in detail the reasons behind aquarium fish eating each other, and what you can do to avoid the issue.

Why Do Aquarium Fish Eat Other?

Here are three of the most common reasons why aquarium fish end up eating each other.


Have you noticed several fish corpses in the aquarium? Chances are they any one of them was infected and transferred the disease to its inmates.

Unfortunately, not every disease is obvious, when it comes to fish. That is why it is better to have your fish quarantined before bringing them to your tank.

Maybe a fish was sick before coming to the tank. Failing to cope with the new environment, it might have passed away.

The rest of the fish may have feasted on its corpse and caught the disease the dead fish carried.

Also read: How to Dispose of Dead Fish (the right way)

The infection will take its toll, and while some might survive, most of them will lose the battle.

Chances are that the weak and perishing will also be eaten by the remaining fish.

To avoid any such incident, it is better to quarantine your fish before moving them to a community aquarium.

Also, spend some time to notice the behaviors of different fish, when in a community setting.

To make things easy, record their behaviors in a journal. Also, keep track of the ammonia, nitrate, ph level, and temperature in your fish tank.

In the journal that you have maintained, record the behaviors of the fish, every time you add a new fish to the tank.

You must also observe and document their behaviors when you make changes to their tank. These changes may include, adding new toys, rocks, plants, or air pumps to the aquarium.

If you have everything in place, you are less likely to lose a fish. In case something bad happens, you will know the culprit behind it.

Opportunistic Nature

Another reason behind fish eating other fish is their opportunistic nature. Being opportunistic is nothing new, and is common among several other creatures.

The moment food presents itself, they are meant to take a bite. It’s ironic, isn’t it?

A fish that used to be their best pal is now their food, just because it’s dead. A lot of it has to do with the food chain.

So, how can you determine the fish that is killing those around it? In most cases, there is an obvious killing fish that is easy to identify.

Sometimes, there could be an otherwise friendly fish, which might have a taste for munching on other fish.

However, a series of deaths doesn’t necessarily motivate a fish to eat another fish.

It could be due to disease, change in temperature, or water pollution in the fish tank.

As discussed in the previous paragraphs, the only way to avoid such events is to monitor the fish tank closely.

Be aware of what conditions you need to avoid for minimizing the chances of any fish losses.

New Fish in the Tank

Every new fish that you bring to your tank is closely monitored by others around it. Older fish tend to scrutinize every move the new fish make.

It may sound strange, but even in a peaceful fish tank, the pecking order exists. Just like any other species of animals, in fish, territories are marked.

Therefore, the existing fish tend to keenly observe the new fish and the space they occupy.

If the fish feels threatened in some way, they will come up with a plot to eat up the new fish.

Keenly observe the changes in behaviors every time you bring a new fish to the tank.

If you notice slight aggression, you might as well place the new fish in another aquarium.

You can even make decorative changes in the fish tank.

This will allow the fish to find new space for themselves, and mark their territories. Try to provide maximum hiding sports to the fish.

By doing so, you can add more plants and rocks to the aquarium.

If the aggression persists, you should move the aggressor to another aquarium.

Water Issues

The addition of a new fish to the tank can spike the amount of ammonia and nitrate in the aquarium.

This issue isn’t uncommon, but it can surely create a biological imbalance within the water.

Because of being caught and moved to a new place, the new fish feel stressed.

Because of the stress, the fish might succumb even to the slightest biological imbalance of nitrate and ammonia in the tank.

When changing the water, it is easy to shift the balance of the nitrogen cycle in the aquarium.

It is also possible when cleaning water filters or making similar changes.

Things will eventually normalize, but the fish that is already weak and disturbed will succumb quickly to the change in conditions.

In case one of the stressed fish dies, it can become food for the remaining fish. Only by tracking the changes in the water, you can identify the true killer.

When changing the water, try to shift your fish to a clean water container. Allow the water balance to settle, before moving the fish back to their old aquarium.

Aggression in Fish

Sometimes, the reason behind fish-killing each other is mere aggression.

But, before moving towards stopping them from hurting each other, we need to understand the reasons behind their aggressive behavior.

What Triggers Aggression in Fish?

There are a variety of reasons why a fish might kill those of its kind, and you’ll be having a killer in your aquarium.

Surprisingly, some of the most sociable fish can exhibit aggressive behaviors from time to time.

They might not have special territories, but they will surely compete for their social position.

The fight for the position is one of the primary factors that compel fish to indulge in aggressive activities.

However, if the group is large enough, and has the right balance of males and females, peace might prevail.

One of the most common territorial fish is the cichlid. It tends to use its marked territories for feeding, raising its kin, and even for mating sessions.

The real problem lies in the size of your aquarium. When there is less space in the tank, the fish will have a hard time marking their territories.

As a result, they engage in aggressive competitions and also end up killing each other.

What starts as mere intimidation or harassment, can quickly turn into something nasty, depending upon the competitiveness of the fish.

Here are some ways to identify the symptoms of aggression in fish.

How to Identify and Prevent Aggression in Fish?

Before you start maintaining an aquarium, you should be well aware of the aggressiveness of your fish.

If you have witnessed violent acts among your fish, you can understand that they don’t start killing, right away.

They start slow by biting a fin or two. By doing so, a fish warns the other fish to get out of its way.

Tensions grow when the opposing fish doesn’t back off. As a result, tensions continue growing, which can be life-threatening for the weaker party.

The victim in this duel can be found towards the back of the filter. In most cases, the weaker fish might be panting heavily, and its color will also darken.

If the fighting gets intense, the weaker fish will end up with torn fins, injured, and with damaged eyes and mouth.

Because of excessive injuries and wounds, the fish may eventually die, and get eaten up by others.

Now that we have talked about the signs of aggression in fish, let’s talk about what you can do to avoid unfavorable instances.

Form a School

Try to keep a whole school of the same type of fish.

Although it will not cut the events of biting and fighting, it will surely minimize the chances of something unpleasant such as killing.

In a school setting, make sure to balance the number of males with that of the females.

Most fights among fish of the same breed occur because of the mate.

While fighting over a single mate, fish may become extra aggressive, to the point that they don’t even mind killing.

Remove the Aggressor

If you have tried everything but the biting, fighting, and killing don’t stop, remove the aggressor.

If all fails, removing the aggressive fish, and moving it to a new home is the answer to the problem.

This will restore order in the aquarium, and keep other fish safe.


Another smart way of mitigating aggression among aquarium fish is to overstock the aquarium.

The more fish there are, the lesser will be the chances of territorial violence.


An aquarium with colorful fish is a beautiful thing to have in your living space.

However, before filling it up with different types of fish; try to determine how they will react, and whether or not they will coexist.

Lastly, don’t skimp on monitoring the behavior of your fish in a community setting.

If the aggression prevails, make sure to remove the aggressive fish to a new tank.

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