Why Do Fish Chase Each Other?

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Whether you have an aquarium at home or have observed this behavior in the wild, you probably wonder: “Why do fish chase each other? Is it a form of playing, or is one fish stalking the other as prey?” There are several reasons why fish chase each other around a tank, including coupling, bullying, competing for food, and defending territory. Here’s how to determine the specific cause for this behavior.

What Are the Reasons That Fish Chase Each Other?

#1. Coupling

The most likely reason why two of your fish are chasing each other is that they are playfully expressing interest in coupling up. Chasing is a way of flirting for fish, and it could be a sign that these two fish want to create a family together.

This is a natural behavior and nothing to worry about. If you don’t want to deal with a whole school of fish, you might want to separate these two from each other.

But you should know that it will make them sad to lose their playmate, and they will instinctively look for someone else to form a couple, so your resistance may be futile.

#2. Bullying

The second most common reason why one fish will stalk another around a tank is bullying. Aggressive types like fighter fish will find a particularly interesting subject, such as another fish with colorful scales or long tails, to follow around and harass.

This kind of chasing will be noticeably different from coupling because the fish being chased will show signs of distress, such as darting away quickly or hiding. It won’t look like a fun, playful activity for the victim fish.

Also, the stalking fish will nip at the fish it’s trying to bully. Even if you don’t see the biting occur, you may notice evidence of it in the form of damage to the fish being chased, especially on the tail.

If you see one of your fish chasing the other to bully, you need to separate them immediately. In that case, you can try a breeder, or isolation box. Left to their own devices, the bully fish may resort to cannibalism and eat the other fish.

Aquarium Fish Breeder Box, Fish Isolation Box, Hatching Box, Juvenile Fish Spawning Incubator, Water Isolation Net Hatchery (Green Square)

Preferably, you should always research the types of fish you purchase before adding them to your tank to make sure they will get along well with your other fish.

Guppies, catfish, loaches, tetras, mollies, and swordfish are peaceful types of fish that usually work well in a tank together, so seek out these breeds for your aquarium.

If you’re a beginner, you may want to stick to a single type of fish (particularly a peaceful breed, as fighting fish like bettas won’t hesitate to eat their own kind) and collect a school of them.

Most fish, even the friendly ones, prefer being with their own breed, so you could start by placing several of the same breed of fish in your home aquarium. You are unlikely to see the bully-chasing if you select the right kinds of fish.

#3. Competing for food

Another reason why your fish may be chasing each other is that they are competing for valuable resources, especially food.

This doesn’t mean that your fish are starving and that you need to feed them more. Overfeeding your fish can be just as detrimental to their health as underfeeding them. Overfed fish can become sick, lethargic, and even die.

Overfeeding your fish also makes them produce more waste, which dirties their tank faster and can mess with the balance of their ecosystem.

When your fish chase each other, particularly during feeding time, they just want to ensure that they get their fair share of food.

Don’t feed them any extra, but instead, distribute the food evenly in different areas across the tank and watch to make sure everybody gets some.

#4. Defending territory

The final reason why your fish are chasing each other is that they are marking and defending their territory.

Fish, like people, need a little personal space sometimes. Fish cohabiting in the same tank will find a place to nest, especially with a partner, and claim it as their own. When other fish come too close to the nest, the territorial fish will chase the intruders away.

Don’t confuse this with bullying behavior. Defending territory is perfectly normal and isn’t necessarily a sign that you need to separate your fish.

You can tell the difference between marking territory and bullying by how the territorial fish behaves. A fish defending his nest won’t nip at the intruder fish unless he absolutely has to.

He also won’t follow the other fish around the tank but instead will stay close to the nest to defend it. This usually happens when the tank space is too small.

Additionally, fish become extra-territorial when mating. They will defend their partner against advances from other fish, and if they are defending eggs, they may become more watchful and aggressive.

Especially if you have peaceful fish breeds, they will most likely understand that they are being chased away from someone else’s property and will move on quickly.

Just be careful not to remove a fish from the tank that is defending eggs. If anything, you should use a separate tank for the pregnant fish and their fry.

Bottom Line

If your fish are chasing each other around the tank, there are several reasons that can explain this behavior. They may be mating, fighting, competing over resources, or marking their territory.

Keep an eye on your fish to figure out which reason your fish are chasing each other and separate them if needed. In most cases, though, a little chasing is normal, natural behavior and not a cause for concern.

About the author

Li-ran B.

Li-ran has been taking care of fish since he was a young kid and considers himself a self-proclaimed aquatic hobbyist at heart. What started as a simple childhood curiosity quickly turned into a full-fledged passion. Currently, his new obsessions are nano aquariums and glowing fish tank decorations.