Home > Articles > Freshwater > Aggression in Fish

Aggression in Fish
by Brett Miller

Most aquarium fish adjust to their surroundings and live peacefully with almost any other type of fish. As long as their sizes are compatible and they are non-aggressive they will get along fine, but there are other fish who will kill any fish you put in with it. I can hardly believe my eyes when I read the true account of a bunch of domesticated Oscars (Astronotus ocellatus) making a meal of a household cat and taking a good size chunk out of their owner's arm. Read a few stories similar to this one and one quickly comes face to face with the potential for viciousness in these peaceful-looking fish. My Oscar just sits there and wouldn't bite my finger if I put it in his mouth. This example perfectly illustrates my conclusion that the amount of aggressiveness in domestic fish depends directly on how they are treated during their lives.

My interest in fish began when I visited my friend's house, and his brother showed me his big fish and large aquarium. I started with a little half-gallon bowl and soon upgraded to a ten-gallon tank. I began with mostly community fish (non-aggressive), but after many died I soon looked to the more hearty variety in Cichlids (a family of fish which are more aggressive than most freshwater fish). That was about two years ago and now I have eight tanks, for a total of 149 gallons that houses 27-30 fairly aggressive fish, including a pair of Red Bellied Piranhas (Serrasalmus nattereri), feared by many to be the most aggressive of all freshwater fish. In the wild they are very vicious, but are not found to be so devastating in a aquarium.

I am currently a member of the Lane County Aquarium Society, which contributes greatly to my knowledge of fish. I am also the webmaster and secretary of the club, which entails designing a web page and posting various pieces of information from the meetings.

Size of Fish:

The size of any given fish is usually a good indicator of whether he will dominate the tank, or be dominated. Usually a peaceful ("peaceful" is how I characterize such fish when they are not tearing each other apart) Cichlid tank is filled with Cichlids that are all (generally) the same size. This is true in most of the cases that the average aquarist would observe, but with the highly aggressive fish that I possess, extremely large fish can be put with fish the size of smaller feeder fish and coexist just fine.

The two Piranhas in my tank have cohabited with a two-inch Flying Fox (Epalzeorynchus kallopterus) since I have owned the tank. Slowly, the Piranhas have grown much larger than the Flying Fox. The Flying Fox is even considerably smaller than the fish I feed them. The Piranhas greedily eat the two to three-inch Comet Goldfish (a.k.a. feeder fish; Carassius auratus), and the Piranhas even allow the Flying Fox to nip at their fins. Inherently the Flying Fox is not an aggressive fish, so this behavior by both the Piranhas and the Flying Fox is very surprising.

In the aquarium directly to the left of the Piranhas is a tank containing an Archer fish (Toxotes jaculator). This fish gets its name from its ability to spit at its prey, such as bugs that are on a leaf a good distance away from the water, knocking them into the water where he is able to eat them. Also in this tank are seven aggressive Pearl Cichlids (Geophagias brasiliensus). The Pearl Cichlids are all under one-inch in length, while the Archer fish is approximately five inches long. The Archer fish is a very calm fish and normally would be no match for a Pearl Cichlid, or any Cichlid its own size, but because in this case the Archer Fish is so much larger than the Pearls he is able to coexist perfectly. As the Pearl Cichlids mature, however, I will be forced to move them into a tank with fish their own size as well as similar aggressive behavior.

In yet another tank in the same room are two aggressive fish, a Green Terror (Aequidens rivulatus) and an Oscar. The Green Terror, whom I call Balboa because his person-ality traits are similar to that of the movie character Rocky Balboa, is about five inches long, whereas the Oscar is almost 11 inches in length. They get along fine and only occasionally fight. When Balboa presents himself as a threat, Oscar thinks about fighting but backs away, even though he is much bigger than Balboa.

Territorial Behavior:

Territory plays a large role in the attitude of fish toward their fellow tank mates. A few months ago I had a ten-inch Jack Dempsey (Cichlasoma octofasciatum) in the same tank as Balboa and Oscar. He was put in the aquarium before Balboa became so aggressive (refer to "Size of Fish") and when Oscar was king of the tank. My brother decided to put a rather large Jack Dempsey into my tank, because the large Pacus (Colossoma bidens) in his own tank were going to kill the Dempsey. Hours after Jack Dempsey was moved into the tank Oscar began having jaw fights with Jack. Jack Dempsey was obviously losing and I had to do something, but I lacked the necessary tank space to move him into another tank. Upon the advice of a friend (Technical Advisor Luke Deese), I rearranged the tank in an effort to confuse the Oscar into thinking he was in a new tank. This was intended to force the Oscar to reestablish his territory. With the larger Jack Dempsey in the tank, Oscar hopefully would be unable to establish immediate dominance. This worked, and the two fish then got along for a month or so. During this time, the Jack Dempsey became very comfortable with his surroundings.

Jack Dempsies in general, are much more aggressive than the Oscars, and this Jack began to show it. Jack Dempsey bit a big chunk out of the head of a 14-inch Clown Knife (Notopterus chitala), another fish who was in the tank and was usually a neutral fish when it came to dominance in the tank. Needless to say, he didn't live long after the attack. Not long after this, Jack began terrorizing Oscar and all the other fish in the tank, so I was forced to get rid of him. I took him to another person's house where an extremely aggressive Buttikoferi (Tilapia buttikoferi) the same size as Jack, was living in a fairly big aquarium.

Being in an Aquarium Versus the Wild:

The territorial battles are made so fierce (refer to "Territorial Behavior"), not only because these fish have been put in an aquarium which is much smaller than their original habitat, but also because they are being housed with other fish from all over the globe. Under natural circumstances this would never occur. The aquarium hobby has only been around for part of this century and there are constantly new fish entering the hobby, so the chances of two species that have never seen each other before meeting (in an aquarium) is quite good.

The introduction of two previously unfamiliar species to each other could explain the lack of aggressive behavior the Piranhas express toward the Flying Fox (refer to "Size Of Fish"). My Piranhas have been fed Goldfish (the most common fish fed to Piranhas) for a long time, but maybe, because they have never cohabited with a fish such as a Flying Fox, they do not know how to treat it.

I have never experienced damage to any of my tanks due to fish aggression, but many people have. Reactions to being in an aquarium have lead to tanks being utterly destroyed.

One aquarist had an Oscar who totally wreaked havoc on his aquarium. The fish even tore out the under gravel filter plates ("UGFs").

For amusement he would tear up plastic plants, move stone mounted driftwood around the tank, and eat heaters. I had a power filter and UGF's in the tank, and had to take the UGF's out. He tore up the tubes and pulled up the plastic plates. He also broke tank hoods (where the lights sit), a cross brace on a 55 gallon...(Edelman)

This type of behavior is definitely not normal, but it goes to show that when fish are put in an unfamiliar environment, anything can happen. Although I do have an Oscar who continually latches onto the end of the heater (because he has nothing better to do) he never damages the heater besides putting a few scratches on it.

Fish Response to Humans:

In the wild, fish are not confined to a small space, and people cannot always stare at them, whereas fish in aquariums are forced to deal with humans. Each fish I have usually reacts in a unique way, but there are a few different general responses I have observed.

One response is hiding and trying to get away when someone gets near the tank. My Piranhas are a good example of this. They are some of the most aggressive fish I own, but when I approach the tank they go berserk,hitting the sides of the tank at high speeds trying to get away. If I put my hand in the tank they become even more timid (the feeling is mutual!) and get as far away as they can. They only swim in the back of the tank, so as to stay away from people who might be in the room. They only eat when nobody is in the room, but then (and only then) they will devour anything I put in the aquarium.

There is almost a complete opposite reaction when it comes to Wayne Mah's Black Piranhas (Serrasalmus niger). Mah has studied various species of Piranhas for a number of years, and in his studies he never once mentions the slightest timidness when Piranhas are around people (Mah), which could be a reason for their illegality in Oregon and several other states.

Another reaction altogether is demonstrated by the way one of my brother's Pacus (refer to "Size of Fish") goes to a certain spot in the tank when he puts his hand in the aquarium. That Pacu enjoys being stroked, and in this way my brother is able to always pet him (Miller). These Pacus will even come to the front of the tank as though they want attention any time someone enters the room.

Some fish, however, try to attack their owner from inside the tank. One man found that wherever his Jaguar Cichlid (Cichlasoma managuense) could see him, no matter the distance, he would attack him by hitting the side of the glass very hard. When he put other tanks next to the one with the Jaguar in it the Jaguar would try to go after the other fish. His owner finally moved the other tanks away in fear that the Jaguar would break his tank, because he hit the glass so hard (Freshwater and Marine Aquarium Magazine).

I experience this same kind of behavior in Balboa, the Green Terror. If I put my face up to the tank he flares out his gills to make himself look bigger, shakes, and charges the glass. I do this back to him and he continues to do it until I leave, or at least back off. He displays this same type of behavior with the other fish, an Oscar, that lives in the aquarium with him. The Green Terror is not big enough to damage the tank, but in the future I may need to take precautions so he does not destroy the aquarium.

Having Different Tank Mates:

Growing up, a small fish can survive in a tank with other large, aggressive fish. It can hide and eat sparingly, so as not to draw attack from the bigger fish. The smaller fish may either perfect its hiding habits or assimilate the aggressive behavior of the larger fish by watching and learning. These smaller fish cohabiting with larger fish will also be more likely to grow up to be more aggressive than their mentors. The ones that hide a lot, however, usually die from a disease or sickness brought on by stress. It is very important to select the proper tank mates to ensure the health of a fish.

Isolation:

The isolation of a fish has a drastic affect on his aggressiveness. A fish loses interest in just about everything and will do nothing if left alone without tank mates. I had an Oscar who I called "Big O" because he was bigger than the other Oscar I have. (The other Oscar is previously mentioned in "Territorial Behavior", and is known just as "Oscar.") Big O would rest on the gravel and lean up against the side of the tank for hours at a time. He only moved around when he saw me and recognized me upon my entering the room but still, because he was alone, he did little. Most fish, however, are not as smart as Oscars (refer to Appendix I) and are unable to recognize their owner, and so nothing excites them. Living a non-interactive lifestyle becomes habit with less intelligent fish, so they lose most or all of their aggressiveness.

An isolated fish will not be mentally able to defend itself when put with another fish who has not been isolated and, therefore, has retained its aggressiveness. For that reason it is not a good idea to isolate a fish. Only one exception comes to mind when it is appropriate to isolate a fish in its own tank, and that is when a fish is so highly aggressive that it would be considered cruel to put it with any other fish.

Many people end up buying such "tankbuster" fish, either because they think the fish is beautiful, or because they think their fish are equally aggressive and will not be bothered. When they get the fish home they realize it is much more aggressive than they thought and are forced to return it to the pet store. Some fish have been returned to pet stores up to ten times.

One Red Terror (Cichlasoma festae) was sold four times in one summer.

We isolated it in a 90 gal. because it killed any and all potential tankmates [sic]. The fish was very beautiful, thus many people were interested (McNeill).

Dr. Wayne Mah (refer to "Fish Responses to Humans") has studied the Black Piranha and has found that this fish will not coexist with any other fish. No matter how much bigger the other fish is, it will fight until that fish, or he himself, is dead.

...the Black Piranhas [sic] most infamous qualities [are] unbelievable aggression towards all other fishes [sic], remarkable strength and power and unyielding desire to eliminate all and every possible co-habitant it may come across, including larger foes than itself...(2)

Mah even put his Black Piranha in with other large and highly aggressive fish to prove his point.

...So far none has survived though each one had presented different methods of defence [sic] and attacking responses, but in the end, the Black Piranha always ended victorious.(4)

It is with this extremely aggressive type of fish that isolation becomes inevitable, because nothing will stop the monsters.

Being in Groups:

Fish of the same species have been known to find safety in numbers and, therefore, present more aggressive qualities when around other fish of their own kind. My four Red Devils (Cichalsoma labiatum) will all gang up on a Goldfish, killing the fish. They then proceed to eat the feeder fish piece by piece,because none of them are able to eat a whole one in one bite. One of the Devils is bigger than the other fish in the tank so to prevent injury to the smaller fish I proceeded to move him into a bigger tank. In his new tank, he no longer is able to kill and eat a Goldfish and hides most of the day. Seeing this behavior, it can be concluded that fish feel safe and, consequently, more aggressive in groups.

Another good example of this is when a group of small fish intended to be food for a larger fish, kills the assailant.

I had one very aggressive 5inch Piranha that would not only kill other fish that I put in the tank, but he would torture [sic] them by biting out their eyes...One day my brother came home with three 2-3inch Jack Dempsies [sic] so that the Piranha would have to work for his food but to our suprise [sic], when he went to eat one of the Jacks the other two attacked him and...started to eat him alive...(Smith)

The Jack Dempsies had no right to win that fight, but because they felt so determined within a group of their own species, they ganged up against the Piranha and thus prevailed.

Hunger:

One thing that makes almost any fish aggressive is hunger. When I feed my fish, the larger fish chase away the smaller tank mates in order to get more of the food. It is rare that one fish kills another fish its own size because it is hungry, but sometimes when food is scarce it does happen. Fish that have larger appetites than other fish their own size often eat more because they are livelier and they need the food to sustain their energy level, and this liveliness can sometimes be translated into aggression.

Only once did I have an extremely lively fish who ate little. He was a Pangasius Catfish (Pangasius sutchi), called Louie. Almost all his life he either played dead or swam constantly toward the side of the aquarium, as if trying to escape. I fed him little, but his energy level remained high; however, other fish I own eat large quantities of food and they are not nearly as active.

Space in the Aquarium:

A big problem that many aquarists face is that their fish out-grow their tanks, and they are unable to get a larger aquarium because it is too expensive. In the years to come I hope to get a tank upward of 150 gallons to accommodate my growing fish. The stress resulting from fish overcrowding makes them prone to disease and attack by other fish who are determined to make room for themselves by killing their tank mates. One Oscar killed its tank mates and attacked everything else in the tank. The owner of the overgrown pet fish said, "I think most of this aggression was due to the limited tank space and my own ignorance..." (Sunn)

Due to overcrowding, fish are made into killers even when they are a species that normally would not demonstrate such behavior. This problem is not rare. In fact, some of the tanks I own contain fish that belong in a bigger aquarium. If I do not get a larger tank, it will most likely lead to the death of the fish. If fish within the aquarium hobby are bred with accentuated aggressive features, through neglect, it is possible that in generations to come they will become excessively violent.

The Importance of the Correct Diet:

There are many different types of food you can feed an aquarium fish. It is not as necessary to feed peaceful fish a varied diet as it is to feed aggressive fish good diverse meals. The most common foods for aggressive fish are pellets, worms, feeder fish (usually Goldfish), beef heart, and dry fish food. The use of a wide variety of food causes fish to grow faster, as well as become more aggressive.

Feeding Goldfish to aggressive fish can present some disadvantages. My fish know that when a Goldfish is put in the tank, it is food. Once they get accustomed to chasing down and eating feeder Goldfish, they sometimes exhibit that aggressive behavior toward other fish in the tank.

Another reason for feeding a fish a diverse diet is because it is more healthy for them. The growth of some fish will be stunted if they are fed the same food continually (Aquaria Central).

Another problem with consistently feeding one type of food to a fish is that the fish can become addicted to that kind of food. When food addiction occurs, the only way to get a fish to eat something else is to starve it into submission (Edelman).

Sometimes feeding the wrong food to a fish brings out qualities in a species that it would not normally possess. Pacus are aggressive herbivores, but most people who own Pacus do not know this and feed them what they feed their other aggressive fish. In the wild Pacus eat fruit that falls in the water from overhanging trees, but in some tanks they are forced to hunt feeder Goldfish. This causes Pacus, and other fish, to be more aggressive than they ever would be in the wild.

Importance of Fish Health:

A major component of keeping a Cichlid healthy (which is demonstrated by lively and aggressive behavior) is keeping the aquarium water clean and free of disease. Keeping a tank clean would not be a problem if it were not for two things: the amount of money it takes to buy the proper equipment; and, the time required for proper general maintenance of an aquarium. It is necessary with some tanks to do continuous water changes, which involves siphoning water into a bucket and refilling the tank with fresh water via the bucket. The process takes a great deal of time and sometimes the fish are not very cooperative. Quite often a fish will bite when a person's hand is the tank, occasionally drawing blood.

Steps to maintain health are of the utmost importance if lively, vigorous fish are to be kept. Fish who are unhealthy because of dirty tanks or disease will inevitably die, or at least never be the same. I have a blind Jack Dempsey who does nothing all day. I will have to dispose of him soon, because he is living a pointless life. He used to be very aggressive, even coexisting with fish ten times his size, but because of my negligence he acquired the disease Popeye. He lost his vision, and his personality.

Conclusion:

Many elements contribute to whether a fish becomes a killer or a docile fish. It is important to treat fish correctly and to help them cope with being in an aquarium, as opposed to being in the wild. Many factors must be closely monitored for each and every fish in order to have a healthy and successful aquarium, the most important being the selection of tank mates, diet, and a clean disease-free environment.

Regardless of the species, unnatural aggression in aquarium fish is almost always, in one way or another, merely the result of being in an aquarium. As an aquarist, it is difficult to judge what will happen when two immensely different species of fish are put together. Putting one fish that is ten times the size of another fish in the same tank, may result in the small fish becoming more aggressive as he is forced to deal with the bigger fish, or it may result in the little fish becoming lunch.

In general, my research demonstrates that the amount of aggression in most freshwater aquarium fish depends directly on how a fish is taken care of during its lifetime. The unpredictability of domesticated fish will probably remain a mystery. Who is to know when a fish will go ballistic, and kill all its tank mates, forcing its owner to isolate it? The potential for randomness keeps me interested and gives each one of my fish a unique personality.