HomeAquarium FishWhy Do Aquarium Fish Eat Other Fish?

Why Do Aquarium Fish Eat Other Fish?

Since the first artificial aquarium was constructed in the mid-1800s, fish have been known to attack and eat each other. It has even been a popular topic for study by ichthyologistsmarine biologists, and hobbyists alike.

The reasons that fish become aggressive towards their tank mates are numerous and often nuanced. Here’s a glance at some of the numerous common causes as to why fish eat other fish:



This is perhaps the number one reason that people go out to buy new pet fish. Each goldfish requires about 10 gallons of water per inch of their body length (so a five-inch long goldfish would need 50 gallons). 

Since many varieties of goldfish and other types of domesticated fish grow quite large, it can be challenging to find housing for them in a house with, let’s say, a rather cramped living room. 

A single goldfish seems innocent enough when it is just a tiny fry swimming around the pet store tank, but within a few months, it will be outgrowing its small home and will likely become aggressive towards other fish that share the same water.

Not only do regular old fish require more space than they appear to need from aquarium shop tanks, but cichlids (i.e., African and South American fish) and loaches (i.e., Asian carp-like fish) require even larger spaces—they can grow up to two feet long!

It does not matter what class or variety of fish you decide to keep in your home aquarium; make sure you do your research and go over the maximum recommended size per fish.

Overcrowding will not only cause aggression between tank mates, but it can also cause stress as these larger fish feel like they are in a constant state of territorial conflict with other fish who share their waters.

Water Quality


Fish require clean water to survive. 

If there is not enough circulation or filtration, one might experience a spike in ammonia levels that can cause a lack of oxygen, leading to suffocation. 

A lack of dissolved oxygen can trigger a vicious cycle where stressed fish will produce more waste than eventually contaminates the water further. 

Even if the right amount of filtration is provided for however many fish you have, some aquarium fish are more territorial than others, so it is essential to have multiple hiding places for fish being bullied or harassed by other tank mates.

Fish can also become sick if the water quality isn’t kept at a proper level. 

Poor maintenance causes an unhealthy buildup of ammonia and nitrites, which can lead to infection in the fish that currently reside in the water and reason for new fish to contract diseases during their introduction into your aquarium.

For many species of freshwater fish, pH levels should be kept between 6.5-8.0

Fish produce waste continually throughout their day, releasing acidic waste into their environment that lowers the pH level in the water and makes it unfit for sustaining forms (mainly tropical fish). 

If the water is too acidic for them, you can find yourself with many dead fish on your hands.

The nitrite level in the water should be zero at all times since it is more toxic than ammonia and cannot be filtered by biological or mechanical means. 

Nitrates are only harmful in higher quantities which might not always be avoidable if you have multiple large aquariums with lots of fish living in them simultaneously. 

If you choose to overcrowd, however, even this might not be enough filtration power to keep the nitrate level down.

Fish may become stressed if they are introduced into an environment with different water parameters than what they are used to (either naturally occurring or artificially created by additives). 

For example, if your goldfish lived in a freshwater aquarium all its life and is then moved to an ocean-like environment, it will most likely freak out, try to jump out of the tank, and become very sick from the change.

Overcrowding also makes it difficult to feed your fish properly. 

It is crucial to get the proper ratio of food per fish right—too much or too little can lead to malnutrition and death. 

So if you have half a dozen hungry mouths to feed in your goldfish bowl (which, by the way, only holds about five gallons), how are they supposed to eat enough when there’s not even room for them all in front of the floating flakes?

poor diet can cause abnormal coloring in fish; lack of proper nutrients can cause loss of colorspotsraised scales, and ulcers.

It is essential to remember that some species are more aggressive than others, so while goldfish may do fine with other goldfish species or even with bettas (a popular freshwater aquarium fish), trying to mix these same species up might not turn out so well.

Introduction Of A New Fish In The Tank


Other inhabitants of a fish tank will closely monitor every move the new fish make. Older ones tend to scrutinize their space and behavior more than younger ones. 

Still, even peaceful tanks have order-in-systems with established hierarchies that players must abide by or face the consequences like being eaten alive!

It would help if you were keenly observing the changes in your fish’s behaviors when you bring a new one to live with them. 

If they start displaying aggressive or territorial behavior, move that individual over as soon as possible because it will only worsen from here on out until someone gets hurt.

You may want to try making some decorative changes in the fish tank. 

This will allow your fish more space and create new hiding places for them, which can lead them away from fighting with each other over territory (and help you avoid an aggressive situation). 

Maybe moving their habitat is worth considering if that doesn’t work out or if things get worse than expected.

Also Read: Molly Guppy Hybrid – Is it Possible?

What Causes Aggression in Fish

If you have a large tank that you would like to stock with many different types of fish, then it’s best to know which ones will play nice together—otherwise, your tank could resemble an episode from Jaws: the Revenge.

The most common form of aggression between fresh and saltwater fish is territorial fighting during mating periods. 

Some species are more regional than others, but in general, when it comes to mating rituals, nearly all species hate each other.

Another common form of aggression occurs when there is competition for habitat or territory (i.e., food and shelter) since this means there is a limited amount available.

Other causes of aggression to watch out for include:

  • Incompatible tank mates: This can be caused by certain fishes’ need for water parameters that others cannot provide.
  • Filtration issues: If your aquarium’s filtration system is not powerful enough to compensate for the number of fish you have living in it, you could end up with a buildup of ammonia and nitrites, which will make your fish sick.
  • Contamination: If the water becomes too dirty, fish will start dying.

Food can sometimes be another cause of aggression. Fish feed in a pecking order (just like chickens at the farm), and it’s not uncommon to see your tank’s big guy bullying everyone else away from the feeder when he thinks his belly is full.

What Can Be Done To Avoid Aggression From Fish?

  • Do not overcrowd your tank.
  • Get plants for your tank.
  • Add insect-repelling or tasting chemicals to remove bugs that might fall into your aquarium.
  • Maintain proper filtration levels by providing sufficient space per fish if you’re tight on cash.
  • Look out for symptoms of disease development within the group; this may be indicative of poor dietpollutionparasites, etc.
  • Don’t be afraid to remove sick fish from the tank if needed (after properly containing disease).
  • If you’re considering buying fish for your aquarium, research their compatibility beforehand.
  • Do not place small goldfish in a big bowl; they will eventually overpopulate because of their lack of space, leading to filtration issues. Ideally, they should be placed in an aquarium since their minimum size is 10 gallons. This requirement goes up exponentially with each additional goldfish (e.g., 20 gallons for two goldfish, 40 gallons for 4).
  • Goldfish are omnivorous, which means they eat almost anything that falls into the water, including algae wafers and pellets specifically made for freshwater fish– try alternating between both types to avoid overfeeding.
  • Do not forget to watch your fish’s food intake; smaller fish will die if they’re not getting enough to eat, even though the large fish are gulping them down like nobody’s business. For this reason, it is best to keep small fish separated from larger ones.
  • Purchase plants for decoration rather than consumption since goldfish also munch on plant leaves.
  • Smaller aquariums only house 1 type of fish because there’s not enough room for more.

Treat your fish well, and the bond you share with them will be strong– after all, they won’t drag you into an aquarium shop by your ankles.

Do Aquarium Fish Eat Other Fish?

Yesaquarium fish may be adorable in the eyes of many, but if you are not careful enough, it might cost your fish’s life as fish eats other fish.

There is no exception because it depends on the type of species to whether the species of fish eats other fish or not. If you wonder and inquire, most fishes that live in the same tank will eat each other.

Many people are tempted to buy an aquarium to keep fish at home. However, before buying the aquarium, one must know how big their pet will be when fully grown up. 

The aquarium will be suitable to keep for those who are willing to take care of many fishes, especially those living loners such as cichlids.

There are types of fish that can’t hurt one another, although they live together in the same aquarium. 

One example is loaches which have no teeth to tear off their friend’s flesh or scales. However, certain types of them will attack other fish if it feels disturbed or threatened.

The word aquarium fish may be misleading to you. However, the truth is that there are many species of fish in this world, and each has its characteristicsbehaviors, and habits

It is because of this reason that they become so unique from one another. 

Some fish eats other fish, while some fish do not eat each other or eat much less.

Fishkeeping Expert
Fishkeeping Expert
With years of collective fishkeeping knowledge, we are pleased to share the tank maintenance tips and fish care advice we've picked up along the way. Fishkeeping Expert is home to all fishkeeping tutorials and guides that will help you keep your fish healthy.


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