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Why Do Fish Chase Each Other Around The Tank?

why do fish chase each other around the tank

Why Do Fish Chase Each Other Around The Tank?

If you have a community tank, the thought of “Why do fish chase each other around the tank?” has probably crossed your mind at least once. As a responsible fishkeeper, it’s normal for you to worry. Is constant chasing dangerous? or is it a telltale sign that you need to make some changes in your setup?

Having multiple fish of different species share an aquarium without doing a bit of compatibility research beforehand is a recipe for disaster.

While not all chasing around the tank is dangerous, you can end up with a few casualties if things get out of hand. Some types of chasing are quite natural in a community tank.

The good news is you can usually tell if the chasing is normal/expected or a situation you should defuse immediately. Stressed-out, bullied, harassed, or hurt fish will definitely need your intervention.

Reasons Fish Chase Each Other

fish nudging each other

So, why do fish chase each other around the tank? Learning to distinguish between when fish chasing each other is natural/to be encouraged, and when one (or more) fish is getting harassed will help you tweak your tank setup to keep all fish safe, healthy, and happy.

Here are the 6 main reasons why fish chase each other around the tank:

1. Territoriality

There are several types of territorial behaviors, depending on what triggers a fish’s natural instincts:

  • Territoriality of solitary fish species

There are some species of fish that simply prefer their own company and can react negatively to having their space trespassed by fish species that are more social or have a playful nature. 

In this case, a solitary fish chasing a more playful tank mate won’t turn aggressive unless the loner gets cornered and feels in danger.

  • Territoriality during breeding

Female fish, and males of some species, will get territorial when their tank mates get too close to their breeding site. This is normal behavior and is a sign that the future parents will protect the eggs.

If this type of back and forth between a spawning pair and other fish gets too aggressive, you should move pregnant fish/the eggs to another tank.

  • Territoriality towards newcomers

A certain hierarchy (the one you might not even notice!) usually establishes in a community tank after each fish gets its turn to test boundaries and get tested by their tank mates.

A newly introduced fish can be chased around in a display of territoriality. This type of chasing should subside once all the fish get a turn to interact with the newcomer, and it finds its place in the hierarchy.

2. Showing dominance

Displays of dominance among members of the same species can look like fish chasing each other around the tank. Shoaling species usually have an alpha male that tries to show dominance just like it would do in the wild.

Guppies are notorious for this type of chasing, but they shouldn’t look aggressive or cause any fatalities.
There are instances where males fighting for dominance can lead to injuries, but those situations are preventable.

If a shoaling group of fish is too small, the dominant male can cause some damage, as chasing can turn into fights. In this case, adding more fish of the same species is the easiest fix.

The alpha male will have less time to fixate on just one or two individuals, which should determine it to cut back on bullying.

3. Incompatible tank mates

Every fish care-guide comes with a list of tank mates, compatible species, and/or a list of fish species you should avoid pairing your fish with. This type of information is usually shared by aquarists because they’ve seen just how graphic the consequences of tank-mate incompatibility can be.

Putting predators and fish that they consider prey in the same tank, and expecting things to go smoothly just because they are pet fish, is a rookie mistake.

Aggressive fish will harass less-energetic fish species to the point where constant chasing and nipping can cause permanent damage/death.

If you plan on setting up a community tank, you can avoid the chasing that comes with incompatibility by making sure to check if all the fish you plan on getting have a compatible temperament.

You can find workarounds if you already have incompatible fish living in the same tank (more on that later in the article!), but if all else fails, separating them in different tanks might be the only solution.

Aquarium Industries has provided some nice compatibility charts for common aquarium species – check out the links below!

Freshwater Fish Compatibility Chart 

Saltwater Fish Compatibility Chart 

4. Competition for food

One of the biggest challenges for novice fishkeepers when setting up their first community tank is figuring out a good feeding routine that takes into account every fish species’ dietary needs/habits.

Competing for food can cause fish to chase each other around the tank in a not so playful manner. If food is scarce, or if you have too many fish feeding on the same type of food, there’s bound to be some aggressive chasing going down.

Seeing as aquarium fish are pet fish, you should avoid having them fight over resources. Weaker, or shyer, fish can face starvation and even cannibalism in extreme cases. 

Providing enough food/nutrients and tailoring the feeding schedule to meet the needs of all the fish species in your community tank is how you stop fish from competing for food.

5. Mating rituals & courtship

If male fish of the same species suddenly start chasing each other, mating season is just around the corner. This type of chasing is natural and will dwindle as the males figure out which one of them gets breeding rights.

There is one other type of mating-related chasing that can be a bit more problematic. When the male-to-female ratio in a tank is disproportionate, the mating ritual can result in exhausting harassment for the existing females.

Platy fish are the perfect example of this type of mating behavior. They need at least 3 to 4 females per male to keep the mating chase peaceful. So, if you notice that most of the chasing is targeting females, you should definitely consider adding more female fish to balance out the situation.

If you notice females chasing males, that’s either a sign that they are ready to mate or that they are acting in self-defense. Females will chase away non-dominant males and over-zealous pursuers during courtship.

6. Overcrowding

You can easily overcrowd a community tank, and that can trigger fish to chase each other in a mix of territoriality and self-defense. An overpacked aquarium can make even the most social fish species feel stressed and act-out in consequence.

When does overcrowding become a trigger for aggressive chasing?

  • When a tank is too small for the number/size of fish living in it;
  • There’s not enough open-space for swimming;
  • There are too many add-ons crammed in one tank (tank gear, décor elements, pebbles, plants, breeding cages).

See why fish chase and fight each other (cichlid edition) here:

Fish Chasing Each Other Is Normal Behavior

First things first: fish chasing each other around the tank is normal behavior!

Not all chasing should be viewed as a “situation” that you have to fix.

Of course, there are plenty of chasing behaviors that can look like playful roughhousing to you while actually being a stressful situation for the fish getting chased.

Playful chasing between fish is their way of interacting. They do it to form social bonds, to establish hierarchies, and to find mating partners. All-natural behaviors that you shouldn’t get in the way of!

Chasing that turns into fighting or bullying is a completely different story.

How To Stop Fish From Fighting

how do fish kill each other

When a chase between fish turns into a fight, you’ll typically notice the aftermath of any aggressive behavior after the fight ends. Injuries are proof that your fish aren’t just chasing each other but that they will continue to fight unless you make some changes in your tank’s setup.

You should work on stopping fish from fighting if you notice any of these signs:

  • nipped or split fins
  • chunks of scales missing
  • scratches
  • an atypical change in one individual’s behavior
  • visible injuries that require quarantining the wounded fish

The most efficient way to stop fish from fighting is to address the factors that trigger aggressive behavior. Whether the fights happen over mating rights, dominance, food, or territory, you’ll need to either make the appropriate changes (see above!) or separate the two offenders in different tanks.

How To Stop Fish Bullying

Any fish tank-mate compatibility chart will tell you that semi-aggressive/aggressive fish should be kept with similarly tempered fish species. Stray from this recommendation, and bullying is guaranteed!

Bullying can start off as a harmless chase, but when one fish is getting targeted repeatedly, things can take a turn for the worse.

There’s only so much aquascaping you can do to offer the victim of bullying enough hiding spots/refuge areas. An unrelenting bully can injure (and ultimately kill) its victim.

Pregnant females, already injured fish, newcomers, and shy peaceful fish tend to be victims of bullying when sharing a tank with feisty fish species.

The only 100% efficient way to stop an insistent individual from bullying other fish is to move it to another aquarium, where its tank mates share the same temperament.

Territorial Fish Species

Territoriality can look different depending on which fish species you’re observing.

While solitary fish will chase away any fish that trespasses its space boundaries, some fish will treat a territory the same way they treat their dominance over the females.

That’s when 2 males of the same species will fight to the death (like male betta fish do).

Males that show territorial aggression will even chase and attack, males of a different species if they have similar physical traits and colors.

Popular territorial fish species:

  • Arowanas
  • Angelfish
  • Betta fish
  • Oscars

Semi-aggressive/Aggressive Fish Species

why are my fish nipping at each other

Semi-aggressive fish can share a tank with equally tempered fish species if the aquarium is large enough for the number and size of the fish you’re considering.

Having one male of every species is a good preventive measure you can take to keep aggressive chasing at a minimum.

Popular semi-aggressive fish species:

  • Angelfish
  • Gouramis
  • Betta fish
  • Green spotted pufferfish

Aggressive fish are best kept as a single species in an aquarium, as they typically are ferocious predators that will make a quick meal out of any potential tank mate.

They need plenty of room for swimming, feeding, and “personal” space even when sharing a tank with members of their own species. Caution is advised!

Popular aggressive fish species that can’t safely share a tank with other fish:

  • Silver Arowana
  • Red tail catfish
  • Paroon shark
  • Alligator Gar
  • Wolffish
  • Snakehead fish
  • Vampire fish
  • Goliath tigerfish

Even some nano fish species can be aggressive. See the example of Lamprologus ocellatus here:

Aquascaping Tips To Prevent Fish From Chasing Each Other

If you feel like the atmosphere in your community tank is getting tense, even though you have compatible tank mates sharing an aquarium, you can use aquascaping to help keep the peace.

When playful chasing turns into bullying or fighting, the fish that feels threatened will instinctually search for a hiding spot. You can help tone-down aggressive chasing by making a few aquascaping changes to your tank’s setup.

Here are 3 basic tips to help diffuse the tension and put some distance between the bullies and their victims:

1. Create plenty of hiding spots

Aquascaping using décor elements is more efficient in preventing fish from chasing each other if there’s a consistent strategy behind it.

Your goal should be to create mini-territories that the aggressors can claim, keeping them out of every other fish’s business.

To help the victims of aggressive chasing, you can create extra hiding spaces using rock caves, driftwood, clay pots, or decorations with plenty of small crevices.

Creating territories is especially helpful if there’s a lot of chasing between males of the same species going on. You can use synthetic plants to delimitate territories for each male, and you’ll see a significant drop in fighting for dominance.

2. Rearrange tank decorations

If you find that your initial aquascaping strategy isn’t working, and there are more fish getting harassed/bullied than not, you can try rearranging tank decorations.

Look at what your community tank might be missing. Is there too much open space that doesn’t get used for swimming because the fish feel too exposed? Are your fish overcrowding a certain part of your tank?

If that is the case, you simply need to spread out your décor elements and/or plants to break up the tank into more sections.

If chasing results in injuries because fish keep running into decorations that have a rough texture or sharp edges, you should consider switching to smooth stones or live rocks.

3. Add plants as refuge areas

Live aquatic plants are essential for a peaceful community tank! Fish will need a break from chasing, even if it’s done in a playful non-aggressive way. Having a “forest” of tall plants to retreat into when overwhelmed will also help your fish be more confident about swimming in open spaces.

Tall background plants make awesome sheltering areas for any fish that might get harassed or bullied, including injured fish, pregnant females, and even vulnerable juveniles/newcomers.

List of tall plants that make excellent hiding/refuge areas:

  • Water wisteria
  • Green Foxtail
  • Hornwort
  • Amazon Sword
  • Italian Vallisneria

Quick FAQs

are my fish fighting or mating

Why are my fish suddenly nipping at each other?

If the peaceful bunch of mild-mannered fish in your community tank suddenly start to nip at each other, there’s a chance your tank’s conditions or water parameters are out of balance.

A stressful atmosphere in the tank can result in fin nipping.

In this case, you should monitor the fish and look for signs of disease, poor diet, under-oxygenated water, poor water quality, etc.; and make changes accordingly.

Are my fish fighting or mating?

Chasing during mating/courtship will typically happen in cycles and dwindle when females are not ready for breeding.

Fights between fish will carry on forever unless you intervene (separate tanks/aquascaping). And unlike chasing during the mating ritual, aggressive chasing will leave marks on the body of the two fish fighting.

Are my fish fighting or playing?

Fish that engage in shoaling/schooling behavior can get so much into each other’s space that you’ll be wondering if they’re fighting or playing.

If there’s lots of rapid swimming, without signs of aggression or targeted chasing, you can rest assured that your fish are simply playing.

Fighting fish will most often only have 2 individuals interacting at once, and it will look intentionally aggressive.

Feel free to separate fish that are constantly getting into fights, but don’t act on the first display of aggression.

Establishing dominance, winning mating rights, and laying out boundaries for newcomers, can all require a one-time fight that’s followed by only peaceful interactions.

Conclusion – Why Do Fish Chase Each Other Around The Tank?

Fish chase each other for a wide range of reasons: some triggered by their natural instincts, and some being brought-on by a tank setup that needs some tweaking.

Monitoring fish, and holding off on immediately intervening, is the best way to tell if the chasing that’s going on in your tank is natural or if you need to step in and protect an individual from aggression.