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Why Do Aquarium Fish Chase Each Other?

Why Do Fish Chase Each Other?

Fish chase each other for a variety of reasons, such as defending their territory, establishing dominance, competing for food, and mating. Even fish that are typically docile fish may chase others because of constant stress. This could be due to incompatible tank mates, poor water conditions, or an overcrowded tank.

When fish chase each other, it may look like they are playing, but this behavior is a warning sign. Something is wrong in your tank, and it is time to do a little investigative work and find the problem.

This article will cover the most common reasons that fish chase each other and offer potential solutions. Remember that it is impossible to know what a fish is thinking or feeling, and the best we can do is go by our best guesses and make changes we hope will work.

It is important to understand the care needs and temperaments of every fish you intend to stock before you bring them home. You can avoid many common problems if you simply research fish before you buy them.

Many African cichlids are territorial and may chase each other.

Many African cichlids are territorial and may chase each other.

Territorial Fish

Some fish are territorial. They may lash out at others of their kind, others of the same sex, or they may not want any other fish near them. This is especially problematic in small aquariums.

When a fish establishes a territory, it will chase away any other fish it sees as a threat. This is not only stressful for the fish being chased but also for the fish doing the chasing, which is always in fight mode. Stress is one of the biggest reasons fish die too soon, so we want to eliminate it as much as we can.

Some fish look to establish dominance over others of their species or even fish of other species. This is seen especially in African cichlids, but several other freshwater aquarium fish may exhibit dominant behavior as well.

In small tanks, there may not be enough space for this complicated interaction. Sometimes, even a 55 or 75-gallon tank isn’t big enough. The result is stressed fish that might hurt each other, or at the very least die too soon.

How to Manage Territorial Fish in Your Tank

Research fish before you bring them home and understand their needs. This way you avoid stocking aggressive, territorial fish to begin with.

Another thing you can try is rearranging tank decorations periodically so that territorial fish lose their bearing. This can sometimes reset the interactions between fish in your tank, or at least offer a reprieve while the aggressive fish reestablishes its territory.

Poor Tank Conditions

Fish chasing each other could be a sign of poor water quality. You may also notice fish swimming up and down the tank glass. We call this glass surfing, and it is a sign that something may be wrong in your tank.

When the tank gets dirty, fish become stressed. Poor water conditions can affect gill health and lead to a host of diseases. Animals experiencing pain or discomfort are more likely to lash out. Even tiny fish like neons get nippy at each other when the water conditions are bad.

Remember that the water does not need to be visually dirty for it to harm your fish. Waste chemicals such as ammonia build up in the water and harm their health. Eventually, you may see visible signs such as increased algae or even murky water.

How To Keep Your Water Parameters Under Control

The solution is to keep up with tank maintenance. Perform regular water changes and vacuum your gravel.

You don’t have to guess if your water is safe for your fish. Learn to use a freshwater testing kit and test it regularly. I've always used API Freshwater Master Test Kits. These kits are inexpensive and easy to use, and they will tell you exactly what is happening in your aquarium.

Keeping your tank clean goes a long way toward keeping your fish healthy.

Livebearers like guppies sometimes chase each other due to their mating behaviors.

Livebearers like guppies sometimes chase each other due to their mating behaviors.

Mating Issues

Some fish love too much. This behavior is especially prevalent in livebearers like guppies, platies, and swordtails. One or more males might chase a female everywhere she goes. It's a mating thing, but when there are too many suitors she can experience a great deal of stress, which may lead to premature death.

It might look cute when your guppies are chasing each other around the tank, but try to figure out what is happening. If what you are seeing is one or more males constantly harassing a female, you’ve got a problem.

How to Deal With Aggressive Mating Behaviors

Stocking males and females in appropriate ratios helps. You will want to aim for three females of each male in the tank. This will cut down on the stress experienced by a single female fish.

Yes, you are going to have to learn to tell the difference between male and female fish. Some species like swordtails are easy (the males are the ones with the swords) but with others, it gets more complicated. Again, a little research can make a big difference in your tank.

Angelfish are predators that may see smaller fish as prey.

Angelfish are predators that may see smaller fish as prey.

Incompatible Tank Mates

Some fish just aren’t meant to live together. A fish that sees another as a threat, for whatever reason, may chase it.

Some fish are docile and make outstanding community fish. Others are predatory fish and may try to chase and eat other fish if they get the chance. Some fish are semi-aggressive, which means in certain circumstances they can present a danger to other fish in your tank.

Often, aquarists bring together fish species from very different parts of the world. Sometimes they can get along fine, but it is important to remember that you are forcing two different animals from very different environments to live together.

How to Choose the Right Fish for Your Aquarium

There are no simple answers when it comes to choosing fish for your aquarium. Once again, it is so important to research fish before you bring them home so you can have a good idea of what to expect. Even when you do your best to make smart decisions, things can go wrong. If you know what to expect, you will experience fewer unwanted surprises.

Remember that big fish eat little fish and you can’t expect an animal to ignore its natural instincts. If you keep a school of neons with your freshwater angelfish, don’t be surprised if you see your angel chasing them. It is better to avoid that situation altogether.

If you realize that some of your fish just aren’t getting along, it might be time to re-home some of them. Often a local pet store might take them off your hands.


Overcrowding can turn otherwise peaceful fish ornery. Tanks with too many fish in them not only deny their inhabitants enough space to live but usually exhibit poor water conditions. This leads to stress and illness in fish, and it may cause them to lash out at each other.

Sometimes schooling fish will chase a sickly fish away from the group. We see this behavior in neon tetras, one of the most peaceful fish you can stock. When the neons are nippy, I always take it as a sign that something isn’t right in my aquarium.

Scarcity of resources is another thing that may make your fish chase each other, and this can occur in overcrowded tanks. The first thing that comes to mind is probably food. Fish that aren’t getting enough to eat may become aggressive around feeding time and lash out at others. That could mean other fish are out-competing them for food and you may need to figure out how to make sure they get their fill. Or, it may mean you aren’t feeding the right food to accommodate every fish in your tank.

How to Avoid Overcrowding in Your Fish Tank

Once again, the best way to avoid overcrowding is to have a good understanding of the needs of each fish you intend to stock. That means doing the research before you bring fish home, not after. Some fish need a lot of space, where others are happy enough in a somewhat crowded environment.

If you already have way too many fish, consider re-rehoming some of them or upgrading to a larger tank.

Solutions for Fish Chasing Each Other

As you have seen throughout this article, the primary way to avoid fish chasing each other is to make smart stocking decisions based on research. A few more things you can do include:

  • Stock livebearers in a 3:1 female to male ratio.
  • Rearrange the tank decorations so that territorial fish lose their bearing.
  • Keep up with tank maintenance.
  • Re-home problem fish when possible.

Depending on your aquarium and which fish you stock, you might expect to see some minor aggression in your fish tank. A fish that establishes a small territory may chase others away, and they more or less learn to avoid that area of the tank. Fish such as African cichlids may appear aggressive while establishing a hierarchy, but that behavior ought to calm down once everyone knows their place.

However, constant aggression such as chasing isn’t good for any fish, including the ones that have nothing to do with the interaction. It creates an ecosystem where every fish is stressed and on edge, right on the verge of fight or flight.

Stress is one of the biggest reasons fish die in home aquariums. In most cases, it is preventable, if you know what to watch for. Fish chasing each other constantly is one of those things.

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2021 Eric Dockett