* Stress: Jack Dempseys are relatively hardy fish, but they can become stressed if they are kept in poor water conditions, overcrowded, or exposed to other stressors. Stress can cause a fish to change color, and it can also lead to other health problems.
* Disease: Some diseases, such as black spot disease and bacterial infections, can cause fish to turn black. If your Jack Dempsey fish is showing any other signs of illness, such as lethargy, loss of appetite, or rapid breathing, you should take it to a veterinarian for diagnosis and treatment.
* Genetics: Some Jack Dempseys are simply born with a black coloration. This is not a sign of any health problem, and it is simply a natural variation of the species.
If you are not sure why your Jack Dempsey fish is turning black, it is always best to take it to a veterinarian for diagnosis and treatment.