[leebca]

WELCOME TO REEF SANCTUARY!

Fish often waste away form either poor nutrition or internal parasites or a combination of the two (assuming the fish has properly acclimated and not poisoned). A large percentage of fishes come through the system and share their intestinal parasites with each other. In the wild these parasites are not that lethal. The fish eats enough good food to feed both itself and the parasites.

In captivity where nutrition is restricted, the parasite can slowly manage to rob the fish of its nutrition and the fish starves to death.

I can't tell from your description what is going on, but the above being as common as it is, is a good place to start.

I advocate that all newly acquired fishes be de-wormed. I am also a strong advocate of feeding natural foods -- marine fishes eat seafood -- of the right kind and quantity.

Here are some posts that may help you out:

First, about proper nutrition:
Feeding Marine Fish and Marine Fish Nutrition
and the form of foods to serve:
Different Forms of Marine Fish Foods

Then, 'de-worming' as I call it:
Strange Excrement from Fish -or- How to be a Super Pooper Snooper

De-worming is easy. It can be done while the fish is still in the display. It is a food additive.