Whenever my fish finish eating they start to change color really black and whenever they want to eat the color look really beautiful , is that a sign of sickness?
What you feed them? Seem it gives trouble after eating or may be too heavy feeding.
Adjust the diet. Feed them with live food.
When the fish turns black, it is something wrong.
You have to observe it closely and find out what is wrong.
Treat them with antibiotic plus salt.