Aggressive saltwater fish come in a variety of shapes, sizes and aggressive traits. Some fish are more protective of territory; some are more inclined to fight over food. Do your research before adding a fish to your aquarium to make sure you're able to provide the environment needed to avoid injuries—or death—among your fish. Learn the signs of aggression and how to limit this behavior in seven fish that are notorious troublemakers.
Signs of Fish Aggression
Are your fish getting along? Watch for these signs of trouble in your tank that may need to be addressed to prevent stress, injury, or fatal attacks.
Active Attack: Among some species, including those in the wrasse family, conflicts commonly involve breeding territory and preferred hiding spots. You will see one fish actively biting or ramming into another fish. This dispute needs to be handled quickly by removing the aggressor or the victim, especially if they are already injured. You can attempt reintroduction after a few days, but the conflict may continue.
Feeding Behavior: In a well designed tank of adequate size to house its fish population, all fish have equal access to food at feeding time. If you notice that certain fish are hiding during feeding time or that one fish consistently takes food away from other fish, you may need to reduce your population or just remove particularly problematic fish. Large fish aren't the only culprits; small fish that steal food from the operculum (gill covers) of larger fish may be causing stress, too.
Charging: While less likely to cause acute injury than active attacking, charging creates insidious stress. It occurs when a seclusive fish attempts to leave its shelter, and a more aggressive fish forcefully swims toward it so that it retreats back into its hiding place. If this happens regularly, and the seclusive fish is unable to leave its home, it will become stressed and sick.
Breeding: Some fish aggressively defend their nests, eggs, and young; others aggressively try to eat others' offspring. These situations are stressful and can result in injured or dead fish. If breeding is not your goal, it's easier to keep one fish of each species or non-breeding pairs.
Limiting Fish Aggression
Many fish from the family groups listed below have similar aggressive tendencies. It is strongly recommended that you research the environmental needs of any aggressive species you'd like to add to your aquarium prior to purchasing it.
In general, you can help limit fish aggression with a few simple steps:
- Add your most aggressive fish to the tank last. This will allow more timid fish to find hiding spots and establish their territory.
- Don't keep breeding pairs of fish. Some breeding fish parents turn deadly in defense of their developing young. Fish eggs and young will also encourage aggressive feeding behavior from other species, creating conflict and stress.
- Add additional hiding spots where shy fish can take refuge.
- When feeding, disperse the food throughout the aquarium to allow well-spaced feeding and discourage aggression over food.
7 Aggressive Species
The following fish are beautiful but potentially troublesome additions to an aquarium. Knowing their individual space requirements and behavioral tendencies can help you incorporate them into your aquatic environment, if possible, or avoid these aggressive fish altogether.
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The clown triggerfish (Balistoides conspicillum) can grow very large—over 18 inches—and requires over 300 gallons per fish. They are vicious nibblers and will decimate hard coral and invertebrates. These aggressive fish require roommates that are as large and "tough" as they are.
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Many damselfish are known for fiercely defending their territory, and the neon damselfish (Pomacentrus coelestis) is no exception. Although they do not need a lot of water volume per fish (about 30 gallons), they will aggressively attack any fish that approach their caves. They have even been known to bite their human caretakers.
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Blue Line Grouper
The blue line grouper (Cephalopholis formosa) is a small member of the monstrous grouper family, requiring a mere 250 gallons (minimum) per fish. These voracious fish eat smaller fish and crustaceans, so they should not be kept with any creatures small enough to fit into their mouths. Crustaceans that stay hidden in tight crevices may be safe if the grouper can not pry them out.
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The goldbar wrasse (Thalassoma hebraicum) is a bright but aggressive addition to a saltwater tank. Requiring 125 gallons minimum, they become bullies when new fish are added to their tank. Once established in their territory, the wrasse will attack any new occupants. To minimize this behavior, wrasse should be the last addition to your tank, so all other fish can establish their territories first.Continue to 5 of 7 below.
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Coral hogfish (Bodianus mesothorax) transition from juveniles with pale pink and black with yellow spots to adults with striking maroon faces, black mid-body bands, and yellow bodies. They can grow up to eight inches and require 70 gallons per fish. They are beneficial to have in a tank since they will clean parasites off larger fish, but they will attack smaller fish and decimate coral.
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Don't let its small size fool you—the banded hawkfish (Cirrhitops fasciatus) is an aggressive tankmate. These attractive fish have a bright coloration and minimal tank requirements of 30 gallons. However, they require a tank with fish larger than themselves, as they will eat any smaller inhabitants including crustaceans.
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Jeweled Moray Eel
As with all moray eels, jeweled morays (Muraena lentiginosa) have two pairs of jaws—and they bite. This species is smaller than many of its relatives, so it is better suited to smaller aquariums (at least 50 gallons in size). As nocturnal predators, eels will ambush fish and crustaceans in the night unless taught to feed during daylight hours. Eels are also escape artists, so make sure your tank has a secure lid.