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Why Do Aquarium Fish Fight? 7 Causes of Aggression in Your Tank

Eric is an aquarium enthusiast with over two decades of experience caring for a wide array of tropical fish.

Sometimes fish fight. It is unfortunate, but aggression in aquariums is all too common. While experienced fish keepers know what to watch out for when choosing fish and setting up their tanks, novices can be confused and frustrated by behaviors that don’t seem to make sense.

When fish chase, attack, or even kill each other, it can make you want to quit the hobby. Each fish species has a specific temperament. Add in variability with individual animals and you might feel like it is impossible to get every fish in your aquarium to live in harmony.

The good news is, fish are not nearly as complicated as people. There are no ulterior motives, emotional hang-ups, or grudges. Fish react to their environment and exhibit behaviors. When fish fight, we can usually pinpoint the problem if we know what to look for.

In this article, you will read about seven of the most common issues that lead to aggression in aquarium fish, as well as some tips on how to avoid, alleviate, or eliminate the problems.

That said; please remember that this is only general advice. Every tank is different, and only you can do the necessary research to choose compatible tank mates and figure out exactly what is happening in your aquarium. You can consider this article a starting point, but it should not take the place of your own investigation.

Reasons Aquarium Fish Fight

Here are some typical issues at a glance.

  1. Incompatible Tank Mates
  2. Lack of Space
  3. Territorial Behaviors
  4. Poor Water Conditions
  5. Predatory Behaviors
  6. Feeding Issues
  7. Illness and Disease

Read on for more in-depth explanations.

1. Incompatible Tank Mates

Some fish don’t go together. As I always say in these articles, the most important thing you can do to make sure all of your fish get along is to do research before you bring them home. Find out what kind of water and space they require, whether they are territorial, if they will try to eat other fish, or if they are simply too aggressive to keep in a community setting.

You need to dig a little into this. Never make choices solely based on what the staff at the pet store tells you. Some are very knowledgeable, but others are not. In some stores, the same staff member that sells fish might also work in automotive or sporting goods. In my opinion, those kinds of stores should not be selling fish.

The point is, while a knowledgeable employee can be a big help, you don’t need to rely on these folks to tell you what is best for your tank. With the massive amount of information available online, you can gather all the info you need on any fish rather quickly.

Fish such as Oscars are highly aggressive and not appropriate for many tank mates.

Fish such as Oscars are highly aggressive and not appropriate for many tank mates.

2. Lack of Space

Overstocking a tank, or choosing a tank that is too small for your fish, can cause stress. When fish are stressed, they may behave in ways that aren’t in line with their usual demeanor. Some may lash out, whether this is because of a perceived lack of resources, or simply because they aren’t comfortable with other fish so close to them all the time.

It is important not to overstock an aquarium. We’ve all been there, especially as novices. It is a lot of fun to purchase new fish and watch them putter around your tank, and it is easy to get out of control. It is an impulse we’ve all had to learn to manage.

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The solution is, once again, to research fish before you bring them home so you can be sure your tank is appropriate for them. Don't rely on blanket advice such as the oft-repeated one-inch-per-gallon rule. Consider each fish individually, and how they fit into the puzzle of your overall tank setting.

If you find yourself in this situation accidentally, you’ll either need to upgrade to a bigger tank or re-home some of your fish.

3. Territorial Behaviors

Territorial fish may not tolerate other fish of their species, of their sex, or any other fish at all. For some, such as betta fish, this need to fight is tied to mating behaviors.

Territorial fish may establish a little spot in the tank of their own and chase away any fish that comes near. Or, they may see the whole tank as their space and constantly pick fights.

It seems obvious that aggressive, territorial fish would pose a danger to other fish in the tank, and that is certainly possible. Less obvious is the stress and hardship on the aggressive fish itself. When an animal is constantly on alert, fighting off intruders, and trying to defend its area, it can take a toll. Stress is the number-one reason aquarium fish die before their time.

The answer is to avoid stocking territorial fish, but if you already have one there may be a couple of solutions you can try.

  1. Break up lines of sight in your tank with plants and decorations so aggressive fish can't see other fish as often.
  2. Some fish keepers rearrange the tank every few days so that the territorial fish is temporarily disoriented.

These are Band-Aids at best. This is yet another situation where researching a fish before purchase makes a big difference.

Male betta fish will not tolerate each other and might fight to the death.

Male betta fish will not tolerate each other and might fight to the death.

4. Poor Water Conditions

When water conditions deteriorate, fish get stressed. When fish get stressed, sometimes they start acting a bit uncivilized. Of course, it is important to keep up with water changes and a tank maintenance schedule. But this issue also goes hand in hand with the problem of overstocking your tank.

In addition to your pretty fish and plants, your aquarium is also home to a bacterial colony that helps to break down waste and keep water conditions safe for fish. When you have too many fish in your tank, or if you add fish too quickly, that bacteria can’t keep up. Levels of harmful chemicals rise, and fish start to feel it.

Not overstocking your tank and staying on track with maintenance both go a long way toward avoiding this issue. You also want to be sure you cycle your tank properly before you even add fish.

Finally, I suggest testing your water regularly so you know exactly what is going on. I recommend the API Freshwater Master Test Kit for its ease of use, but there are many water testing kits on the market.

5. Predatory Behaviors

Sometimes fish eat other fish. There are a few reasons for this, one simply being that many fish are opportunists who will eat smaller fish if they can catch them. Therefore, you always need to be careful mixing tiny fish with larger fish. Big fish eat little fish.

Other fish are aggressive predators who will hunt and eat other fish in the tank. The freshwater Angelfish is commonly listed as semi-aggressive, but it is an ambush predator that will hunt and eat small fish if it can.

Other fish grow too large and their predatory instincts are too strong to keep them from eating or fighting with other fish. Those fish are better off alone in a single-specimen setup, though they can sometimes exist with fish their size.

Research is once again the key. Bringing home a fish only to find out it intends to eat all the other fish in your tank is a nightmare. Learn as much as you can first, so you aren’t surprised.

6. Feeding Problems

If you notice your fish acting aggressively during feeding time, there could be a few explanations.

First, consider if your tank is overstocked, as mentioned above. When you drop food in the tank, it should be possible to spread it out a little so all of your fish aren’t fighting for it in the same area.
Making sure everyone gets fed is another issue. Some fish are timider, while others will dive right in. Slower fish may get bullied by more active fish as they compete for food.

You can try to alleviate this by feeding in several small batches over a short time period instead of all at once. Feed bottom feeders sinking pellets. Supplement the diets of algae eaters with algae wafers.

Some fish need special consideration. For example, Plecostomus do best with a little natural driftwood in their tank for them to rasp on. Denied what they need, they can lash out at feeding time.

Plecos are typically docile but known to lash out at feeding time if their needs aren't met.

Plecos are typically docile but known to lash out at feeding time if their needs aren't met.

7. Illness

Sick fish can become aggressive with others in your tank. They can also be the target of healthy fish. We often see this in schooling fish, where the rest of the school will nip at or chase away the sick fish. This seems to be a natural mechanism to prevent illness from spreading to the rest of the group, but when it happens in your tank, it can be disheartening.

Many fish keepers quarantine new fish in a separate tank before adding them to the main aquarium, which is a good idea if you can do it. This might keep illness out of your tank.

Otherwise, it is smart to learn and watch for the signs of illness in tropical fish. When you spot something odd, your actions will depend on the sickness and the prognosis. Options range from treating the whole tank to removing the sick fish and helping it recover in a quarantine tank. Unfortunately, it is sometimes necessary to euthanize a very sick fish.

Manage Aggression Before it Happens

As an aquarium owner, it is understandable to feel helpless when fish fight. You can’t exactly jump into the tank and break it up. But, as you can see from this article, there are ways to manage aggressive behaviors in an aquarium.

Preparation and research are key to avoiding unpleasant situations. Knowing the temperament of a fish, compatible tank mates, feeding preferences, and appropriate tank conditions can prevent situations that encourage aggression.

Proper feeding and tank maintenance can keep conditions from deteriorating to the point where fish lash out at each other. Quarantining fish, either before adding them to your tank or when illness appears, can stop strong fish from bullying sick fish.

As I always say, the more you can learn about aquarium care, the better you will get at this fish-keeping thing. Mistakes will happen, so don’t beat yourself up. Just keep on doing the best you can.

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.

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