What to Know About Jack Dempsey Fish

Medically Reviewed by Vanesa Farmer, DVM on January 05, 2023
4 min read

The Jack Dempsey fish is a freshwater species of cichlid known for its feisty nature. Despite this, they are popular among aquarists due to their attractive iridescent colors. Learn about some of the common characteristics of Jack Dempsey fish and how you can care for them.

Jack Dempsey fish (Cichlasoma octofasciatum) are freshwater fish known for their active and aggressive nature. They are named after the famous American heavyweight boxer Jack Dempsey. These fish are also called Jack Dempsey cichlids.

Jack Dempsey fish are native to Central America. They’re commonly found in tropical regions in Mexico and Honduras, especially the bodies of water that border the Atlantic ocean. Their natural habitats include drainage ditches, canals, creeks, and other still bodies of water with rich vegetation.

Because these fish are easy to breed, some aquarium hobbyists in the United States have accidentally introduced Jack Dempsey fish into the wild, where they thrive as an invasive species. Wild populations can now be found in South Dakota and Hawaii as well as in peninsular drainage areas in Florida like the Tampa Bay and the Everglades.

The Jack Dempsey fish has a flat, oval body with 17 to 19 spines on its top fin, eight to 10 spines on its bottom fin, and a rounded tail fin. It can grow to about 25 centimeters long, but it’s usually smaller than that on average — about 7 to 20 centimeters. 

Jack Dempsey fish colors are one of their most well-known and appreciated characteristics. Two gray-black bars run from the tops of their heads all the way forward to between their eyes. The head, body, and fins of adult Jack Dempsey fish have several rows of bright green, blue, and white iridescent spots, which make these fish look highly attractive. 

Some adult Jack Dempsey fish — nicknamed electric blue Jack Dempseys — have a bright blue color all over their bodies. This is due to a natural mutation in their genes.

Young hatchlings, which are called fry, are naturally camouflaged against predators due to their pale gray color and turquoise dots.

In a home aquarium, the Jack Dempsey fish's lifespan can be more than 10 years. As they are primarily freshwater fish, their tolerance to saltwater is very low. However, studies indicate they can live in bodies of water containing very low amounts of salt — up to eight parts per million. They thrive in temperatures between 22 and 30 degrees Celsius.

Female Jack Dempsey fish can lay as many as 800 eggs in a clutch. Both males and females protect the eggs. During the spawning season, when females release their eggs and males fertilize them, they can become extremely aggressive toward other fish.

The protective nature of adult Jack Dempsey fish towards their young is considered one of the primary reasons why their population booms any time they’re introduced to new environments.

When choosing a tank for your Jack Dempsey fish, consider covering the bottom with a soft sandy substrate. These fish spend a lot of time near the tank substrate, especially when caring for their young. Avoid using substrates with sharp edges.

Jack Dempseys can be territorial, so their tanks need to be large. As a general rule, it’s best to have 10 liters for every three centimeters of fish length. Jack Dempsey fish are aggressive with other fish species, so it’s best if they’re the only species of fish in the tank.

To make the tank resemble a Jack Dempsey’s natural habitat, plants, rocks, and wooden branches. Plants add oxygen to the water, while the stones and wooden branches offer shelter.

Interestingly, crowding your Jack Dempsey fish’s habitat with shelter options tends to make them less aggressive overall. Some studies show that the aggressive streak in these fish might depend on the availability of resources in their habitat.

To maintain the water temperature between 22 and 30 degrees Celsius, you’ll need a heat pump, and you should also have a good filtration system. Your fish releases ammonia into the water, which is toxic for them. Maintaining the purity of the water in your tank will help your fish live longer.

When changing the water, remove 10% of the water from the tank and clean the filter by rinsing it in this water. Avoid rinsing the filter under tap water, as this kills the beneficial bacteria that may grow on it. If you think your water contains chlorine, use a dechlorinating agent on the fresh water and wait 24 hours before introducing it into the tank.

When introducing the fish into the water for the first time, leave the bag in the water for roughly 30 minutes to avoid temperature shock. The ideal pH range for Jack Dempsey fish tanks is between 6.5 and 8.0, while water hardness can be between 90 and 357 parts per million. Use home kits to check water pH and hardness regularly.

Keep a light source above your fish tank and schedule a light cycle of around eight to 10 hours every day — you can turn the lights off at night. Avoid keeping the tank close to windows as this could increase the water temperature to dangerous levels and also affect their regular light cycle.

Jack Dempsey fish are primarily carnivorous. Their diet consists of anything that can go into their tiny mouths, including worms, insects, crustaceans, mollusks, crayfish, algae, small aerial insects, and even small fish. 

Some Jack Dempsey fish also seem to enjoy sunflower seeds. You can check if your fish likes them, and if they do, include them periodically in their diet as a treat.