Why Don’t the Fish in Aquariums Eat Each Other?


It’s a question that’s been puzzling fish owners for years: Why don’t the fish in aquariums eat each other? Some people think it has to do with the pH levels of the water, while others believe it’s because the fish can see their reflections in the glass. But what is actually responsible for this unusual behavior?

The fish in aquariums don’t eat each other because they lack the mouth morphology to feast on others. Most fish kept in aquariums are not carnivores, or the other fish can’t fit in their mouth, let alone their stomachs.

In this article, I’ll discuss why fish in aquariums don’t eat each other. You’ll also know situations when fish in aquariums can eat each other. Keep reading to learn how to ensure fish safety in your aquarium.

Can Fish in an Aquarium Eat Each Other?

Fish can eat each other in an aquarium if the opportunity presents itself. However, because most aquarists know this, they organize tanks according to fish size to prevent them from eating each other. 

Aquarists ensure all fish in the aquarium are almost the same size to avoid a situation whereby large fish eat the small ones.

The following is a discussion on why the fish in aquariums don’t eat each other:

The Size of Tank Mates Prevents Fish From Eating Each Other

In a food chain, it’s common to have larger predators eating small prey. The same happens to fish in an aquarium. If you keep large fish, especially cannibalistic fish like Salmon, with small fish species, the chances of the smaller fish being eaten are high.

Most aquarists are aware that large fish can eat small fish. Therefore, they know it’s best to keep fish of the same size in the tank. For instance, it’s common for most aquarists to keep goldfish alone in one tank because they are of the same size and species. In such a case, these fish won’t eat each other.

Appropriate Spacing Stops Fish From Eating Each Other

Fish in aquariums rarely eat each other because they’re spaced appropriately. The right spacing depends on the fish species. For example, Saint Francis Veterinary Center says that a 20-gallon (76-liter) aquarium is suitable for only two goldfish.

NBC News discusses that overcrowding can turn fish into cannibals due to stress. In fact, one of the leading causes of stress in aquarium fish is overcrowding.

Most fish become aggressive when overcrowded due to the struggle for space and other resources like food. The aggression level also depends on the fish species you’re keeping.

Some fish species are known to be more aggressive than others when overcrowded. For instance, cichlids are some of the most aggressive aquarium fish. They’ll start eating each other if you keep them in an overcrowded tank.

Fish Species Combination Impacts the Potential for Fish Cannibalism

Aquarists consider fish species before keeping them in the aquarium. In most cases, tropical freshwater fish are mixed in a tank. Most of these fish are labeled as “community fish” because they are peaceful and will not eat each other.

Some common community fish include the following:

  • Guppies
  • Plecos
  • Catfish
  • Corys
  • Loaches
  • Mollies
  • Tetras

Mixing freshwater and saltwater fish can cause aggression, which may lead to fish eating each other. Such a combination is prone to territorial fights, especially during mating seasons. Therefore, the best way to avoid such scenarios is to mix fish species that can coexist peacefully.

How To Prevent Fish in an Aquarium From Eating Each Other

Your fish can eat each other in an aquarium if you don’t take the necessary precautions. The following are some of the best ways to ensure your fish don’t eat each other:

Avoid Overcrowding

One of the main reasons why fish eat each other in an aquarium is overcrowding. Overcrowding can lead to aggression and stress, which are some of the factors that can make fish eat each other.

It’s important to note that the correct stocking density depends on the fish species. For example, small fish like guppies can be kept with many in one tank. On the other hand, large and aggressive fish like cichlids need more space and should not be kept in an overcrowded tank.

A rule of thumb, however, is to have one inch of fish in one to two gallons (3.8 to 7.6 liters) of water.

Keep the Right Fish Species Together

Some fish species are known to be aggressive and cannibalistic. Therefore, you should avoid keeping such fish in a tank with other fish species. For example, keeping goldfish with cichlids is not advisable because the latter are aggressive and can eat the former.

Final Thoughts

Aquarium fish don’t eat each other because they are appropriately spaced, and the suitable fish species are kept together. Appropriate spacing ensures that fish get enough resources like food. This eliminates competition that can make fish aggressive. Furthermore, aquarists avoid combining different species in a tank to prevent fights and cannibals that can eat other fish.

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