I have a black skirt tetra but its not black anymore,whats wrong?

Usually, a reason for this is that it is getting older and the scales are paler. Another good reason is that the lighting at the top of the fish tank. the lighting, like a tan, makes the scales fade or change, so you shouldn't leave on the light for over 5 hours, which is still bad. i turn mine on for about 20 min. and that's it. When you go to sleep that's when your fish should go to sleep. Your tank might also have parasites, which can cause either minor consequences or very severe ones. i suggest going to buy some aquarium salt, but don't put in too much because putting in too much can also cause it to become sick. It can also be because your fish is stressed or the balance of your tank has gone wrong. I suggest buying some ammonia test and ect. Go to the local pet store or PetSmart cause they have a lot of advise and products. If nothing is wrong with your fish and all that happened was the change of color in its scales then i don't think anything darastic is going on, but if it is acting weird or you see signs of illness or parasites go to your local vet to get it checked out. There are also many places that offer surgery for dropsy, bloating and many other disease you may encounter in the future. I hope this help. :P