Set the temperature of the aquarium thermometer to 78 degrees F three days before purchasing the Jack Dempsey cichlid. If the fish is already in the tank, gradually scale back the temperature, dropping two degrees at a time. This will help prevent shock to not only the Jack Dempsey cichlid, but any other fish in the tank.
Adding an aggressive fish such as a Jack Dempsey cichlid to a tank takes proper preparation. Aggressive fish will not only fight with others like them but weaker fish. The Jack Dempsey cichlid can live in the same aquarium as mildly aggressive fish. Due to their aggressive tendencies, these fish do not work well in small aquariums. Fortunately, there are methods to take to lower the aggressive tendencies of the Jack Dempsey cichlid. These methods, such as keeping the temperature comfortable, can make for a more harmonious tank.
Check the pH level of the water. The Jack Dempsey cichlid needs a pH level between 7 and 8, according to The World of Jack Dempsey Cichlids. Adjust the pH as necessary. To increase the pH, add acidic aquarium chemicals. Use chemicals to lower the pH, if needed. Only use aquarium materials to adjust the pH levels. To know how much chemical to add to the water, read the instructions. The amounts depend on type, current pH level and aquarium size.
Remove any strongly aggressive male fish, like bettas, from the tank. Place these fish in an aquarium away from the tank that will contain the Jack Dempsey cichlid. The fish shouldn't see each other.
Place rocks, caves, plants and other items inside of the tank. Place the cave near a corner of the tank.
Place the bag containing the Jack Dempsey cichlid inside the tank so it floats. Place the bag in a low-populated area of the tank, such as a corner. Use the lid of the aquarium to hold the bag in place. Do not be surprised if the fish darts around the bag. After 15 minutes, open the bag and let the fish swim out naturally. Remove the bag and throw it away.