help

What are your ammonia, nitrite, and nitrate levels? What are his tankmates? Have you attempted to treat with anything? What size tank is he in, and what additives do you put in your water? How long have you had him? Also, does he have any other symptoms (anything on his skin, eyes, etc.)?

Could you please provide the actual numbers, along with your salinity and pH readings? These are extremely necessary in determining what's wrong with your fish.

^ exactally...

~ocean

Can you please provide specific readings of Ammonia, Nitrates, Nitrites, Salinity, PH, as well as temp. Additionally we really do need a photo. We really can't go further with such little information - especially considering that fact that you claim to have had this fish for years with the same tank mates, of whom are not suffering whatever he is - that tends to rule out most of what I would have detailed.
Please refer to the Posting guidelines for this area to get a better idea of what you should be posting. We'd love to help, but until we have more info, we could only make guesses, and if your fish is struggling to breathe as you say, and lethargic, the conditions I am thinking all have a high death rate if not addresses immediately.