Why are Goldfish Chasing Each Other?

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Goldfish are cute, handsome, and resilient fish, capable of thriving in both single-species and community aquariums, provided you ensure optimal living conditions.

These cold-water fish are generally easy-going and peaceful, but that doesn’t mean they won’t show their metaphorical teeth at times.

Part of caring for your goldfish comes down to assessing their behavior since it can speak volumes about their state of mind and comfort level. Having several goldfish in the same tank is bound to lead to a variety of social interactions.

Learning how to read them will help you differentiate between playful and aggressive behavior since goldfish will display both, depending on the circumstances.

So, how do you know whether your goldfish are being playful or in a fighting mood when chasing each other?

Today, we will look into the goldfish’s behavior, trying to highlight the main reasons for them chasing each other throughout the tank.

Reasons Goldfish Chase Each Other

Any goldfish community will experience various social interactions, and not all of them are peaceful and friendly. The problem is that novice aquarists may have difficulties telling which is which.

As a result, some of the problems fueling the goldfish’s hyperactive behavior may go unnoticed, leading to even more extensive issues long-term.

Here are the reasons why goldfish may chase or harass each other occasionally:

Breeding Season

Goldfish are egg-layers, so they undergo strict breeding seasons, during which the female lays eggs, and the male fertilizes them via milt.

In the wild, goldfish will reproduce 2-3 times per year, generally, in the warm season, early spring, early summer, and early autumn if the weather allows it. In captivity, it all comes down to environmental conditions.

You can peak your goldfish’s reproductive interest by providing meals richer in protein and increasing the tank’s temperature up to 70-72 °F.

When that happens, the goldfish’s appearance will change visibly. The female will begin to grow eggs, developing the trademark gravid belly, while males will display white spots around the gills and pectoral fins.

Their behavior will also change as males begin to compete over females.

This will lead to 2 outcomes:

  1. Male-on-male aggression – Most people keep several goldfish males in the same tank, which is always recommended for egg-laying species. The female goldfish will produce hundreds if not thousands of eggs over the course of several days. One male can only fertilize a portion of them. Having several males will increase the number of fertilized eggs, resulting in more fry, which is ideal for selective breeders or those selling goldfish for profit. The problem is that males will fight and chase each other during the mating season, often displaying aggression and harassing behavior. This is normal and should only become a problem if the fish’s behavior becomes unhinged. But more on that later.
  2. Mating dance – The mating dance has 2 phases. First, the male goldfish courts the female, swimming around it and flashing its fins to display its intent. Should the female accept the male’s intentions, the second phase begins, which is the male forcing the eggs out of the female. It will achieve that either by pressing the female’s body with its own body against a rock or the tank wall or by poking at its belly. The male will chase the female around, bumping into its belly and rubbing against it until the eggs are released.

All these behaviors are natural during the breeding season, so long as they remain within the acceptable parameters. If males display extreme aggression towards each other or the females, you might want to remove them from the tank.

Otherwise, the victims may experience fish stress which can lead to a variety of secondary issues.

Territorial Instinct

Goldfish are generally not territorial fish. They are considered among the most peaceful tank fish, perfect for larger community aquariums.

However, they can sometimes display territorial behavior if the conditions are not right. This generally happens either if the tank is too small or if goldfish are overcrowded.

Unfortunately, it may be difficult to determine the exact tank requirements for goldfish due to one of this species’ unique characteristics. That’s the fact that goldfish don’t have a set size.

The fish’s size is heavily influenced by how large its environment is. That being said, you can have a 2-inch goldfish or a 2-feet long one if you keep it in a pond.

The goldfish’s type also influences its maximum growth, so you also need to keep that in mind.

Generally speaking, you should prepare a 20-gallon tank for a pair and add around 10-12 gallons of water for each extra fish. These are the recommended measurements, but I figure you can make it work with less space, just don’t overdo it.

Cramming too many fish in the same tank will activate your goldfish’s dormant territorial instincts and pollute the water fast.

After all, goldfish are notoriously effective poop machines capable of dirtying the water faster than you can cleanse it.

You should also save room for plants and tank decorations since these will calm the fish and provide them with valuable resting and hiding spots. So, you can always use the extra space.

Overcrowded Tank

This is exactly the problem we’ve been discussing earlier. Keeping too many fish in the same aquarium will stress out even the most peaceful specimens.

Their fuses will become shorter, leading to territorial fights and food competition, and battles constantly.

Your goldfish may even fight simply due to the stress of constantly bumping into each other.

Cramming your goldfish in an unnecessarily small space will lead to problems like:

  • Higher ammonia levels – Goldfish produce a lot of poop. The more you have in the same tank, the faster the ammonia levels will spike due to the excess fish waste. Ammonia is deadly to goldfish even in small concentrations, and overcrowding your fish will cause spikes that not even the filter can’t handle.
  • Fighting-related injuries – Goldfish can’t really hurt each other since they don’t have teeth and are not built for fighting. But they can poke at each other’s fins which, in time, may cause tiny tears and injuries prone to infections. And since open wounds and dirty waters don’t go well together, your fish may experience potentially deadly fungal, bacterial, or parasitic infections as a result. And the worst part is that most of these conditions are contagious, which can spell disaster in an overcrowded tank.
  • Jumping out of the tank – Yes, goldfish may even leap out of the tank to escape their overcrowded environment. It’s not a common goldfish behavior, but it can happen in extreme cases. You can fix the issue by placing a lid on your tank, but the underlying problem of overcrowding will remain. Solving one of the symptoms won’t address the problem.
  • Fish stress – Fish stress is a common and potentially deadly issue. Goldfish will become stressed when overcrowded, which will affect their immune system and leave them open to infections and diseases. You only need one contagious disease to hit the tank, and your goldfish community is doomed.

As you can see, overcrowding is a more serious issue than most people believe it to be. Keep your goldfish in a large-enough tank and assess their behavior.

If they show signs of stress or appear overcrowded, consider removing some of them to give the others some breathing space.


Goldfish will display a variety of symptoms and behavioral changes when sick. These symptoms generally depend on the nature of their health issue, but some signs are pretty much universal.

These include:

  • Lower appetite or refusal to eat
  • Lethargic behavior, as the fish swims less and appears apathetic and drained
  • Resting on the substrate for extended periods of time
  • Displaying unwarranted aggression towards other tank mates, etc.

Aside from these, the goldfish will also display disease-specific symptoms, depending on the nature of the condition. These include the inflated belly, white spots on the body, signs of fin rot, etc.

However, one of the most damaging signs remains the aggressive behavior towards other tank mates, usually stemming from the fish’s disease-related distress.

That’s because it increases fish’s stress levels and boosts the risks of contagion due to them bumping and poking at each other.

If you suspect your goldfish may be sick, quarantine the fish immediately.

This will provide you with 3 immediate benefits:

  1. Provide the fish with a stable setting – The sick fish can relax and a stable and comfortable environment with healthy water parameters and adequate food. A treatment tank also allows you to set up the water parameters to the necessary values to counter the disease more effectively.
  2. Protect the rest of the population – Quarantining the fish is essential if you’re dealing with a contagious disease. A parasitic infection like Ich can quickly spread to the entire tank with dire consequences for your goldfish population.
  3. Allow you to use medication – It’s never advisable to use antibiotics in the main tank. These can destroy the tank’s biofilm, eliminating the beneficial bacteria and opening the door to unhinged ammonia boosts. Not all antibiotics will do that, but most will. It’s always safer to treat your goldfish in a treatment tank to prevent that.

Feeding Time

Typically, goldfish aren’t aggressive when it comes to feeding time. It’s only when they are overcrowded or the food is scarce that they become more competitive.

This isn’t necessarily a reason for concern. Goldfish have a set hierarchical organization, and they will use violence and intimidation to enforce it.

This happens primarily during feeding and mating, 2 of the activities that inform the fish’s behavior more than anything else.

You may see your goldfish chasing each other during feeding, as the higher-ranked males use force to get access to better and more food.

This isn’t something to sweat about. The problem arises when the violence gets out of hand. Some goldfish may be more aggressive than others, bullying the weaker individuals and depriving them of food.

In that case, you should consider mitigating the situation. Some good options in this sense include:

  • Separate pockets of food – This is a good tactic for larger goldfish communities where food-related fights are more frequent than you’d like. The technique is simple and effective. You either spread out the food all over the water’s surface or provide your fish with pockets of food in different areas of the tank. This will divide the population, hopefully allowing all fish to feed properly.
  • More plantsAdding more plants to the tank is actually a great way of mitigating food-related violence. The plants will break the line of sight between the fish, lowering the risks of targeted violence, where one fish gets all the hate and can’t feed because of it.
  • Remove the aggressor – Yes, sometimes it may come down to simply removing the aggressor from the tank. You have to understand that not all goldfish are the same. They differ in personality, so some may be more aggressive and dominant than others, forcing you to remove them to stabilize the population.

Whichever tactic you may choose to embrace, remember that food-related aggression is rather common among goldfish in small doses.

The problem is that, when going out of line, it can lead to some goldfish starving. Long-term, the situation must be handled.

Do Male Goldfish Fight?

Yes, they do. Even the most peaceful fish species will fight since fighting is a universal language that sets boundaries, establishes hierarchies, and, paradoxically, enforces order.

Interestingly enough, not all male goldfish will fight at the same rates or as viciously. Some may be more peaceful, while others are always ready for war.

In this sense, you should consider not only how hard they’re fighting but why they’re fighting as well. Food and female-related battles are common and don’t raise too much concern for the most part. Unwarranted violence, however, does.

If your male goldfish appear to be excessively violent without a good reason, you may consider removing it from the community if nothing else works.

Do Goldfish Play With Each Other?

Yes, goldfish play with each other constantly, which is typical for any social creature. It’s relatively easy to differentiate between playful and aggressive behavior in goldfish by assessing how they act.

Playful goldfish will take turns chasing each other with jittery movements and only over short distances. Their demeanor is mostly lighthearted, with no mean-spirited poking or fin-chasing.

This is a welcomed behavior since it reinforces hierarchical stability without violence and strengthens the bonds between community members.

If your goldfish play with each other, that tells you that they’re comfortable in their environment and they are healthy and active. Nothing to worry about.

What to do With Aggressive Goldfish?

The answer depends on the reason for their violent behavior. If the fish is sick, provide adequate treatment.

If the violence is food-related, consider some of my previous recommendations. Generally speaking, though, there are measures to adopt that function as universal aggression deterrence methods.

These include:

  • Increasing the tank’s size – Investing in a larger tank will provide the goldfish with more space, minimizing the likelihood of them constantly bumping into each other. This will contribute to a calmer and more stable environment long-term.
  • Add more plantsGoldfish live in heavily planted habitats, providing them with hiding spots, food, and comfort. Their artificial setting should mimic those conditions for the same reasons. Plants like Java fern, java moss, water sprite, Anubias, or crypts are great options for thriving and stable goldfish aquariums. You can also consider various like shipwrecks, castles, driftwood, rocks, or underwater bridges to provide goldfish with awesome hiding areas. These will not only intrigue your goldfish and keep them occupied but will include the tank’s esthetic appeal as well.
  • Remove the culprit – Sometimes, no preventive measures can deter the aggressor, at which point you should consider removing it from the tank. It’s better than allowing it to stress out other fish and destabilize the community.

Fortunately, goldfish are typically peaceful fish, so I doubt you’ll deal with too much aggression from their end.


As peaceful and friendly as they are, goldfish share a similar biological makeup as any other creature.

They can be territorial, playful, and aggressive over food, females, and hierarchical order like any other living being, no matter the species.

Your job is to monitor their population and act if the situation goes awry. Fortunately, you now have the knowledge necessary to handle pretty much anything in this sense.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
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