Why Is My Fish Aggressive?

Medically Reviewed by Amy Flowers, DVM on March 07, 2023
4 min read

Certain fish species can sometimes become aggressive and hard to control. Fish that don't live in the right environment tend to be very aggressive. It’s essential that you know why certain species are aggressive before you add them to your aquarium.

Below are some reasons why some fish species are aggressive:

Fighting for food. Some fish will always compete for food. An aggressive fish will fight off other fish that are perceived to be a threat during feeding sessions. To minimize fighting over food, make sure you spread food evenly throughout the aquarium. Also, try to offer different varieties of food to your fish. This action will reduce fish fighting over one type of food.

Territory marking. Certain types of fish want to create and keep their own territory. Once you introduce an aggressive fish to your aquarium, it will attach itself to a particular section of the fish tank and not allow others near. 

If you notice a fish create a territory and begin to fight other fish, there are several ways to help limit this behavior:

  • Rearrange the landscape of your aquarium before adding more fish. It will eliminate any claims to territory for fish that you add to the aquarium.
  • Make sure there are plenty of hiding places in your aquarium. This will encourage aggressive fish to create their territory in one of these places, and it will give the less aggressive fish space in the aquarium to swim freely.
  • If you have more than one aggressive fish, add them at the same time into your aquarium. The fish will have an equal chance to claim territory.
  • Don't overstock your tank. An aggressive fish can get out of control if it feels crowded.

Fish size. Larger fish may take advantage of their size and fight smaller ones. When picking your fish, consider species that are likely to grow at the same rate.

Illness in the aquarium. Some fish can become aggressive when they are sick. Other fish can become aggressive towards sick and weak fish. Always check your fish for signs of sickness to avoid aggressive behavior. 

Below are some types of fish that are naturally aggressive:

Veil Angelfish. These freshwater fish will always start well as a group. After a little while, the school of angelfish will fight among each other in order to establish which one is dominant. You will need to change their water less often because changing the tank's water less has been shown to reduce aggression in the species.

Jack Dempsey. The Jack Dempsey species are also known for their aggressive behavior, which is why they're named after the famous boxer. The fish like digging into the bottom of their tank. They will also uproot plants and move tiny decorations. Do not keep these fish with smaller ones to keep them from being eaten

Convict Cichlid. The Convict Cichlid comes in various colors like pink, black, and zebra. They are 6 inches long but are very aggressive. The Convict Cichlid attacks smaller fish or large less-aggressive fish.

Red Devil. These freshwater fish are another highly aggressive cichlid. The fish grow up to 15 inches and they like to dig. You will often notice them knock down decorations in your aquarium. When adding other fish to your aquarium with red devils, make sure they can defend themselves. Because each red devil requires 50 gallons of water per fish, there are not very many fish that can be kept with them.

Oscar. Oscars are usually not very aggressive towards other fish, but they are not very friendly to aquarium decor. These fish can grow up to one foot and are quite energetic. To reduce the extent of damage Oscars can do, particularly to important parts of your aquarium like heaters or filters, keep them in tanks with a sump at the bottom. A sump holds additional filters that can be accessed from outside the fish tank. 

Bucktooth Tetra. The Bucktooth Tetra are known for biting different species of fish and each other. These fish use their sharp, front teeth to bite other fish and small bugs. If you like this fish, be sure to get a group of them. Having a dozen or more of them reduces the chance for them to fight each other.

Aggressive fish need to be handled with care. Before you put fish in your aquarium, know if it is an aggressive species. Make a point to separate the aggressive fish from the docile ones to stop bullying and attacks.