Aggressive Aquarium Freshwater Fish

Many freshwater aquarium fish are good community players. However, those listed here can be very aggressive if they don’t provide the right environment. It is important to know which species need the upper hand before adding them to your aquarium. Here’s what type of aggressive behavior to look for and which fish are the main culprits.

Compatibility with freshwater aquarium fish

What does aggressive behavior look like?

There are some specific examples of what to look out for if you think your aggressive fish is harassing other people in their aquarium.

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This behavior is easy to notice. One fish attacks the other and either pokes its head or bites it. Sometimes the danger of an encounter can mean that meeker fish remains hidden for a long time.


Much of the competition in an aquarium is either for space, breeding, or food. Aggressive fish can swim around quickly, eat as much food as possible, and steal it from other fish’s mouths and even gills. To limit food competition, feed a variety of floating and sinking foods and distribute them throughout the tank using your filter spill or a powerhead.


Depending on the fish species, some fish parents take a relaxed approach, while others defend their unborn young with all their might. Expect aggressive parents to mark their attacks and give them plenty of space. They attack all human hands that get too close to their brood.

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    Veil emperor fish (Pterophyllum scalare)

    Christophe Archambault / Getty Images

    As teenagers, these fish can get along well in a group. But one day one of the group will choose the others and will have to be separated. Then someone else starts to be the instigator, then someone else until all of your angel fish are separated. These fish are also very gentle on brood, so be especially careful with farmed fish. They have been known to attack their human caregivers!

    Species overview

    Long: Up to six inches

    Physical Properties: Black, White, and Yellow Marble Pattern; long and thin fins with fine webbing

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    Jack Dempsey (Rocio octofasciatum)

    Mirko_Rosenau / Getty Images

    Jack Dempsey belongs to the group of Central American cichlids and is available in different colors. Given their famous boxing namesake, it’s easy to call these fish aggressive. Jack Dempseys is known to be a passionate speech decorator. He digs into sandy pool floors, uproots plants and moves smaller decorative objects. They should not be kept with smaller fish or they will be eaten. They require a larger tank size.

    Species overview

    Long: Up to ten inches

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

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    Convict Cichlid (Amatitlania nigrofasciata)

    Wiljoj / Getty Images

    There are several types of convict cichlids including zebra, black, and pink. These cichlids are also a Central American variety and like to have many hiding places in their aquarium. Only 6 inches in length, they can be very aggressive towards smaller or larger, less aggressive fish. They breed in pairs and share parental care equitably between husband and wife.

    Species overview

    Long: Up to six inches

    Physical Properties: Several black stripes run across the body. Men have larger dorsal fins than women

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    Red devil (Amphilophus labiatus)

    Josh Beasley / Flickr / CC von 2.0

    The red devil is another species of cichlid that was rightly named for its aggressive behavior. These fish, up to 15 inches long, are wild burrows and require 50 gallons per fish. They are known for knocking over many pieces of decoration. So strategically stack any rocks or branches so they don’t fall 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 fish that cannot defend themselves!

    This fish is often confused with the Midas Cichlid (Amphilophus citrinellus), sometimes referred to as the Red Devil Cichlid.

    Species overview

    Long: Up to 15 inches

    Physical Properties: Stocky with pointed fins; classic red color

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    Oscar (Astronotus ocellatus)

    BravissimoS / Getty Images

    There are many types of Oscars available for purchase. Some of the most common are zebra, tiger, red, or lemon. Although they are all the same species, these fish have been bred over several generations to maintain specific color patterns. These fish are only semi-aggressive towards other fish, but with their tank decor they can be very aggressive. Some fish have been known to smash heating devices that they disagree with. These fish can grow to be very large, up to a foot, and require 70 gallons or more per fish.

    To limit “decor attacks” it is recommended to keep Oscars in systems with a sump. This limits the number of components that must be kept in the main tank.

    Species overview

    Long: Up to a foot

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

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    Bucktooth Tetra (Exodon paradoxus)

    bertrand.sant / Flickr / CC by 2.0

    Don’t be fooled by the “Tetra” name! Unlike its laid back Tetra cousins, the Bucktooth Tetra, also known as the Exodon Paradox, is known for making its neighbors sip. With their sharp incisors, they grab tiny insects and other fish. These fish are best kept in a group of their own species. A dozen or more is best, otherwise they will fight each other.

    Species overview

    Long: Up to three inches

    Physical Properties: Radiant body made of metallic silver with yellow fins, sometimes with orange and red tips

There are many other species of fish that can be aggressive or moderately aggressive. There are many more rarer cichlid species that could be added to this list. If you are planning to add an aggressive fish to your aquarium, keep the following points in mind:

  • Always put the most aggressive fish in your tank last. If they are to be added to a group, put the entire group together.
  • Unless you plan to breed your fish, don’t leave mating pairs together.
  • If you have room to expand your tank or create more hiding spots, do so before moving in to that aggressive new roommate.
  • Limit competition during feeding time by feeding a varied water column diet (sinking / swimming / neutral) and spreading it out with your filter spout or a powerhead.