Aquarium Fish Aggression: Causes and Solutions

Updated: Jun 6

aggressive fish in aquarium

When I sat down to research fish aggression I thought I would find a few articles on cichlids, bettas, and how to separate “bully” fish. And I did find several articles covering these topics. However, much to my excitement, I also discovered decades of research which uncovered the complicated and fascinating, mostly yet untold, story of the emotion known as aggression as demonstrated by fish. Each turn of the page revealed just how nuanced and intricate fish aggression actually is. So many factors affect this behavior in ways I never thought of.

Controlling Fish Aggression

aquarium fish aggression

As aquarists, we want only the best for our fish. This means giving them a natural environment in which to display their natural behaviors. Aggression is a normal behavior in fish and serves a purpose. Aggression is used to defend territory and determine social rank. But when dominant behavior becomes more aggressive than what would be exhibited naturally, resulting in death and injury, we want to curb that behavior.

In order to control overaggressive fish in our aquariums, we must understand what fish aggression actually entails. What are the neurological factors that facilitate aggression, how is aggression displayed, what environmental conditions cause aggressive behavior, and why?

Aggression is a complicated phenomenon made up of many different behaviors, expressed for many different reasons. Territory, food, social status, sex, temperature, disease, time, and rearing environment all have an effect on aggression in fish. Aggression can be expressed by boldness (risk-taking), chasing, mouth-locking, fin-nipping, gill-flaring, and more.

In this article, I will detail the conditions in which a fish may show aggression, and how to keep those conditions to a realistic level so fish do not become overaggressive.

The Angry Brain

Aggression is all about defending resources. That resource could be potential mates, territory, or food. When your fish display aggression in the aquarium, they are doing so in order to defend or secure a resource. The more limited they find that resource, the more aggressive they become. This is why fish become more aggressive in overstocked tanks (Kelley, Magurran, and Garcia 2006).

Aggression is a complicated social behavior, consequently, the mechanisms in the fish brain that control aggression are supremely intricate. Aggressive behaviors are expressed in over 40 genes and eight different interconnected neurological pathways (Filby et. al. 2010).

Aggressive behavior in fish involves four parts of the brain working in tandem. The forebrain, hypothalamus, optic tectum, and hindbrain all simultaneously contribute to the intricate expression of aggression (Fibly et. al. 2010).

To begin to understand aggressive behaviors in fish, it is important to know aggression is not initiated by a single stimulus, and is not displayed by a single mechanism. There are dozens of neurochemicals, stimuli, and behaviors at play. The complex evolutionary pathways which exhibit aggression are a result of the complex array of stimuli, or triggers, that initiate aggressive responses.

Types of Aggression

The two main types of aggression are territorial defense and social ranking. Territorial behavior is exhibited to defend resources. Social ranking behavior is exhibited for mate selection.

Territorial Aggression

Territorial aggression is an umbrella term in regard to triggers, or stimuli for aggressive behavior in fish. Fish do not defend a territory for its own sake. They defend territories for the following reasons: protection from predators, potential nesting sites, availability of food, population of potential spawning mates.

In other words, most of the stimuli discussed are in relation to the defense of a territory for one reason or another.

Territorial aggression is exhibited by any aggressive behavior, such as chasing, nipping, or biting, which revolves around one specific location. You’ll often see the aggressor returning to the same location after engaging in these behaviors.

To resolve territorial aggression, you must first determine what is causing the aggressive behaviors. In many cases, fish become overly aggressive when the tank is overstocked (Kelley, Magurran, and Garcia 2006). Too many individuals triggers resource scarcity aggressive behaviors. It can also be a result of not enough viable territory to be claimed. This can be alleviated by breaking up the tank to provide as many defensible positions as possible, while also preventing fish from sensing the actual number of individuals in the tank.

Of course, the best solution for an overstocked aquarium is to be conscious of how many fish you are adding and prevent your aquarium from becoming overstocked.

If your tank is not overstocked, but overaggressive territorial behavior is still observed, the solution is often lack of a defensible territory. Breeding pairs will defend a territory more vigorously than non-spawning individuals, and should have the space they need to do so.

Another trigger may be food scarcity. Determine if you are underfeeding your fish by observing how aggressive they are when feeding occurs. Increase feeding frequencies, but feed the same amount and record how aggressive your fish act to determine if food scarcity is causing overly aggressive territorial behavior. If no change occurs, try slightly increasing the amount of food, but only if your nutrient levels will not rise significantly as a result.

Lack of cover and tanks which are too bare can result in aggressive behaviors when fish feel they are too exposed to predators. Increasing cover through rocks, driftwood, and vegetation will increase potential territory sites, decrease shyness, and help break line-of-sight from aggressors. If you decide to add more rocks to your aquarium, click here to see which rocks are safe.

Finally, if aggression persists, especially after adding new fish, you can try resetting the territories. This is accomplished by moving rocks and décor around the tank to dissolve established territories and force all inhabitants, new and old, to redefine their territories on an even playing field.

Temperature Dependent Territorial Aggression

Several species of fish have been shown to temporarily increase aggressive behavior when temperature is increased. It is unknown if this is because of increased metabolism and energy requirements, along with decreased resource availability (Ratnasabapathi 1992).

A species of African cichlid, Julidochromis ornatus, (Golden Julie) was shown to increase and maintain its level of aggression with increased temperature from 77.9 degrees Fahrenheit to 84.2 degrees F over an 8 month period. This suggests increased aggression from higher temperatures is chronic and does not dissipate after the fish becomes acclimated to the new temperature (Kua et. al. 2020).

There was also evidence that J. ornatus maintained elevated aggression despite waning health conditions. This counter-intuitive, last-ditch-effort behavior, only decreased overall health faster (Kua et. al. 2020).

Be sure to check your aquarium’s temperature and ensure that it is nearer the lower end of the comfortable range for the species you are housing.

Time Dependent Territorial Aggression

Territorial aggression in fish can also change over the course of a day. At night, when fish can’t see as well, they have a harder time defending larger territories, so they shrink their territory size, effectively making them less aggressive (Valdimarsson and Metcalfe 2001).

Atlantic Salmon (Salmo salar) were shown to exhibit higher tolerance of other individuals at night when the light intensity was close to starlight (0.00 to 0.01 lux (Valdimarsson and Metcalfe 2001). They showed the lowest tolerance and most aggression during the day and at dusk (1.00 lux) (Valdimarsson and Metcalfe 2001).

If you have extremely aggressive fish, even if it's just a temporary fix, consider dimming your lights to below dusk levels of intensity. This could be useful when adding new specimens or when working in the aquarium. There may even be a long term reduction in aggression if you permanently keep your light intensity lower.

Social Ranking Aggression

This type of aggression is a response to the aggressive behavior of other individuals, not available territory. Social ranking is an important behavior in many animals, including fish. It is necessary to determine which individuals are the most capable of survival, and therefore breeding potential.

If you notice aggressive behaviors such as mouth-locking, fin-nipping and chasing amongst a group of conspecific (same-species) individuals, that is not location dependent, it is often a display of determining social rank.

These types of behaviors are normal, and as long as they are not exacerbated by resource scarcity or over-stocking, should be allowed to play themselves out naturally.

Understanding why these social ranking aggressive behaviors occur is crucial to determining what is and is not normal behavior, and what steps can be taken to alleviate over-aggressive behavior.


Social rank in most fish can be split into dominant and subordinate individuals. Zebra danios (Danio rerio) are often studied in this regard in a lab setting. The findings discussed below are in regard to zebra danios studied by Filby et. al. in 2010. Their findings can be applied to most social fish species as they studied the expression of 40 aggression related genes found in all fish and even most mammals.

In terms of social rank and the difference in aggression between the sexes, it was found that dominant males are more aggressive than dominant females (Filby et. al. 2010). Both dominant males and females are more aggressive than subordinate males and females (Filby et. al. 2010).

So far, fairly straightforward and predictable. It makes sense that dominant individuals are more aggressive than subordinates, with males being the most dominant. Where it starts to get interesting is in the fact that unlike the dominant individuals, there is no difference in aggression amongst the sexes in subordinates, i.e. subordinate males and females show the same level of aggression (Filby et. al. 2010).

Furthermore, both males and females show aggression when defending territory. Social rank is a higher determinant of aggression than sex (Filby et. al. 2010). So next time you assume the males will be more aggressive, unless they are the dominant male, think again.

Time Dependent Dominance

Time also has an effect on social rankings. In the same study from Filby et. al. in 2010, they found over the course of five days, the social aggression dynamics changed amongst dominant and subordinate individuals.

Over a five day period, dominant males went from 6 times more aggressive than subordinate males to 12 times more aggressive, while subordinate males decreased in aggression (Filby et. al. 2010). Dominant females went from 25 times more aggressive than subordinate females to only 5 times more aggressive, while subordinate females increased aggressive behavior by 11 times (Filby et. al. 2010).

Also, fish kept in isolation exhibit more intense fighting behavior than fish in established social hierarchies (Early et. al. 2006). If the fish you are adding to your aquarium were isolated from conspecifics (individuals of the same species), they may be more aggressive when introduced to conspecifics in your tank.

Consider this when you add new fish to your tank and observe aggressive behavior. The level of aggression may change over the first few days. Before you make any drastic changes to your tank layout, wait to see how the social dynamics settle out.

Reducing Social Aggression

Like with territorial aggression, you can reduce social aggression by creating new territories and increasing enrichment (cover) in the tank. You can also play with the number of subordinates and dominants in your tank to achieve an ideal ratio. If female dominance is reduced over time, but male dominance increases, consider only adding females. This is easier said than done if sexing the individuals is difficult at the time of purchase.

Researching and mimicking the naturally occurring social dynamics for the species in question is the best way to control social aggression. Most people know not to add two male bettas together, but the social rankings of guppies or mollies can be a little more complicated in larger groups. Pay particular attention to sex ratios. That is the number of males to females in a group. Try to mimic the sex ratios found in naturally occurring populations.

Captive Bred vs. Wild Caught Aggression

Now that we know a little bit about what causes territorial and social aggression, what happens when you change the environment the fish was reared in? It turns out captive bred fish are more aggressive than wild caught fish (Kelley, Magurran, and Garcia 2006).

Captive bred fish have displayed higher aggression than wild caught in both structured (enriched) and unstructured (bare) tanks (Kelley, Magurran, and Garcia 2006). This is due to wild caught fish being preoccupied with foraging behavior, while captive bred were not (Kelley, Magurran, and Garcia 2006). In addition, captive bred fish showed the highest aggression in the most densely stocked tanks (Kelley, Magurran, and Garcia 2006).

All this is to say that overstocking bare rearing tanks to save money makes for more aggressive fish. It may be good practice to mimic the natural habitat of captive bred species when rearing them in order to raise specimens better suited for aquarium life.

Parasite Induced Aggression

This last caveat of fish aggression is not common in the aquarium, but I would be remiss if I left it out of this article due to the fascinating biology of trophic parasites (parasites that pass through the trophic (feeding) levels to new hosts).

An experiment conducted in 2010 showed when rainbow trout were infected by the eye fluke parasite, they increased in aggressive behavior, but lost contests for better territory. With the negatives of increased aggression (injury and energy expenditure), and none of the benefits (better territory), they become more susceptible to predators. This is the intention of the parasite as it has evolved to travel between hosts through predator/prey relationships amongst the hosts (Mikheev et. al. 2010).

Basically, if the parasite infects an individual, then causes that individual to become aggressive to the point of exhaustion, the individual will be much easier to consume by predators, allowing the parasite to reach its target host more efficiently.


Aggressive behaviors displayed by fish can be triggered for many reasons, and are carried out by complex neurological systems. Understanding why your fish is behaving the way it is will go a long way to remedying the situation. Some aggressive behaviors are normal and a necessary function of the aquarium. Defending breeding nests and determining social rankings are acceptable reasons to display healthy dominant or defensive behavior. It is when aggressors become overly aggressive and begin to injure or kill tank mates that we know something is wrong. Usually this means the tank is too small or is overstocked, or there is not enough enrichment or cover. There could also be an improper sex ratio; there are too many males or too few females.

When you're faced with aggression issues in your aquarium, and you’ve determined possible causes, take steps to rectify them. Always begin from a place of knowledge by doing your due diligence before purchasing new species. I don’t believe separating “bullies” or moving aggressive fish to a different tank is a viable solution. These are solutions to a problem that should not be occurring in the first place if your tank is properly set up for the species you plan to keep. In other words, treat the cause not the symptom.

Literature Cited

Early, R.L., et. al. (2006) “Social interactions tune aggression and stress responsiveness in a territorial cichlid fish (Archocentrus nigrofasciatus)” Journal of Physiology and Behavior. 88: 353-363.

Filby, A.L., Paull, G.C., Hickmore, T.FA., Tyler, C.R. (2010) Unraveling the neurophysiological basis of aggression in a fish model. BMC Genomics. 11, 498.

Kelly, J.L., Magurran, A.E., Garcia, C.M. (2006) Captive breeding promotes aggression in an endangered Mexican fish. Biological Conservation. 133, 169-177.

Kua, Z.X. (2020) Water warming increases aggression in a tropical fish. Scientific Reports. 10, 20107.

Mikheev, V.N., Pasternak, A.F., Taskinen, J., Valtonen, E.T. (2010) Parastie-induced aggression and impaired contest ability in a fish host. Parasites & Vectors. 3(17).

Ratnasabapathi, D., Burns, J. & Souchek, R. Effects of temperature and prior residence on territorial aggression in the convict cichlid Cichlasoma nigrofasciatum. Aggress. Behav. 18, 365–372.<365::AID-AB2480180506>3.0.CO;2-E (1992).

Valdimarsson, S.K., & Metcalfe, N.B. (2001) Is the level of aggression and dispersion in territorial fish dependent on light intensity? Journal of Animal Behavior, 61(6), 1143-1149.

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