What is your nitrite and nitrate at? Is it's feces long, white, stringy? Are you familiar with the signs of Brooklynella and oodinium? Do you have cyclopeeze (fish usually will eat this even if they're not doing well)? You may also want to add either garlic, selcon, or vita-chem to the food that it is eating. If you could catch the fish I would also consider giving it a freshwater dip (make sure that the pH and temp is the same as the tank) for 5 minutes. After that time check the bottom of the container for anything. Do you have a quarantine tank?