Aggressive Aquarium Freshwater Fish

Convict Cichlid

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Many freshwater aquarium fish are good community players with fish of similar size and temperament. However, the ones listed here, if not given the right environment, can be very aggressive toward other fish. It's important to know which species will require the upper hand before adding any of these into your aquarium. Here's what type of aggressive behavior to look for as well as which fish are the major offenders.

What Does Aggressive Behavior Look Like?

There are a few specific examples of what to look for if you think your aggressive fish is harassing other individuals in their tank.

Charging/Active Attacking

This behavior is easy to note. One fish will charge at the other and either head butt them or bite them. Sometimes, just the threat of an encounter can keep meeker fish hidden for long periods of time.


Much of the competition in an aquarium is either over territory, breeding or food. Aggressive fish may swim around rapidly, eating as much food as they can and stealing it out of the mouths and even gills of other fish. To limit food competition, feed a variety of floating and sinking foods and spread them out throughout your tank using your filter outflow or a powerhead. Observe the fish while feeding to make sure all the fish are getting access to some food.


Depending on the species of fish, some fish parents take a laid back approach, while others defend their unborn young with every ounce of strength they possess. Expect aggressive parents to tag team their attack and make sure to give them plenty of room. They will attack any human hands that come too close to their brood. It is best to separate spawning fish into their own breeding tank.

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    Veil Angelfish (Pterophyllum scalare)

    Freshwater angelfish

    Christophe Archambault / Getty Images

    As juveniles, these fish can get along together just fine in a group. But, one day, one of the group will start picking on the others and needs to be separated. Then another starts to be the instigator, then another, until all your angelfish are separated. These fish are also viciously protective of their brood, so take extra care with any breeding fish. They have been known to nip at their human caregivers!

    Species Overview

    Length: Up to six inches

    Physical Characteristics: Black, white, and yellow marbled pattern; long and thin fins with delicate webbing

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    Jack Dempsey (Rocio octofasciatum)

    Electric Blue Jack Dempsey Fish

    Mirko_Rosenau / Getty Images

    A member of the Central American cichlid group, the Jack Dempsey comes in a variety of colors. Given their famous boxing namesake, it is easy to associate these fish as being aggressive. Known for being fierce redecorators, Jack Dempsey cichlids dig into their sandy tank bottoms, uprooting plants and moving smaller décor items. They should not be kept with any smaller fish as they will be eaten. They require a larger tank size.

    Species Overview

    Length: Up to ten inches

    Physical Characteristics: Strong facial features; large oval body with long fins

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    Convict Cichlid (Amatitlania nigrofasciata)

    Black/Zebra Convict Cichlid

    wiljoj / Getty Images

    Multiple varieties of Convict Cichlids are available, including the Zebra, Black, and Pink. These cichlids are also a Central American species and like to have lots of places to hide in their aquarium. Only topping out around 6 inches long, they can be very aggressive to any smaller fish or even to larger, less-aggressive fish. They will breed in pairs and share the parental care equitably between male and female.

    Species Overview

    Length: Up to six inches

    Physical Characteristics: Multiple black bands running vertically on the body; males have larger dorsal fins than females

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    Red Devil (Amphilophus labiatus)

    Red Devil

    Josh Beasley / Flickr / CC by 2.0

    The Red Devil is another cichlid species aptly named for their aggressive behavior. Growing up to 15 inches in length, these fish are fierce diggers and require 50 gallons per fish. They are well known for knocking down piles of decor, so stack any rocks or branches strategically so they do not fall down on your fish. There are very few fish that can be kept with the Red Devil, due to their extremely aggressive nature. Do not add any fish that cannot defend themselves!

    This fish is commonly confused with the Midas Cichlid (Amphilophus citrinellus), who also is sometimes termed the Red Devil Cichlid.

    Species Overview

    Length: Up to 15 inches

    Physical Characteristics: Stocky with pointed fins; classic red color

    Continue to 5 of 6 below.
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    Oscar (Astronotus ocellatus)

    Tiger Oscar

    BravissimoS / Getty Images

    There are many color varieties of Oscar cichlids available to purchase. Some of the most common include the Zebra, Tiger, Red or Lemon. Even though they are all the same species, these fish have been bred over multiple generations to have varying color patterns. These fish are only semi-aggressive towards other fish, but can be very aggressive with their tank décor. Some fish have been known to smash heaters they don't agree with. These fish can grow very large, up to one foot, and need 70 gallons or more per fish.

    To limit "décor aggression" it is recommended to keep Oscars in aquariums with a sump. All of the filtration and the heater are maintained in the sump rather than in the aquarium. This will limit the amount of components that have to be kept in the main tank.

    Species Overview

    Length: Up to one foot

    Physical Characteristics: White with black, orange, and red outlines and spots

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    Bucktooth Tetra (Exodon paradoxus)

    Bucktooth Tetra

    bertrand.sant / Flickr / CC by 2.0

    Don't let the "tetra" name fool you! Unlike their easy going tetra cousins, the Bucktooth Tetra, also known as the Exodon Paradox, is well known for nipping at their neighbors. They use their sharp, front incisors to grab tiny insects and other fish. These fish are best kept within a group of their own species. A dozen or more is best or else they will fight each other.

    Species Overview

    Length: Up to three inches

    Physical Characteristics: Bright, metallic silver body with yellow fins, sometimes with orange and red tips

There are many other fish species that can be aggressive or moderately-aggressive. There are many additional cichlid species that could be added to this list. Whenever you are planning on adding an aggressive fish to your tank, keep in mind the following points:

  • Always add the most aggressive fish to your tank last. If they are to be added in a group, add the entire group together.
  • If you are not planning on breeding your fish, do not keep mating pairs together.
  • If you have room to make your tank bigger or make more hiding spots, do so prior to moving in the new aggressive roommate.
  • Limit competition during feeding time by feeding a varied water-column diet (sinking/floating/neutral) and spread it out using your filter outflow or a powerhead.