Cheak levels whats the nitrite and ammonia.
(Not nitrate.) these levels are important for the fishes respitory and overall condition.
(Is this a new tank?)
(WATER CHANGE! WATER CHANGE!) How often do you cheak levels? If water levels are okay how long ago did you add this fish to the tank? Chances are it could be unhealthy from the getgo. Levels would have to be high for a period of time for oxygen to deplete.