Hi
couple of questions:
have you added anything to the tank in the way of new fish, (even fish from one of your other tanks), plants, gravel?
How long have you been housing this particular group of fish together?
Have you noticed any aggression between the fish? Chasing?
What are the water parameters and what is your water changing schedule--every week, month? How much water?
If you've recently added anything to the tank then it's possible that she has a parasite however if none of the other fish are showing symptoms then it's less likely that it's a parasite. If something is off in the water quality then this could be effecting her. Sometimes you'll have one fish who is more sensitive to problems with water quality then others.
My guess here is that she has been fighting or being chased by one of more of your other fish. Just a guess.
I would first off test the water if you haven't already and then regardless of the results do a 30% partial water change daily or every other day for the next week using a good quality dechlorinator--look for any improvements in the fish's skin and fins. Additionally I would try to watch the fish when they don't know you're watching to see if there's an aggression problem. If they know you're there then they may just beg for food.
Okay? And if you can get back with some more info that may help us figure out what's going on
Robin