First things first - detectable Nitrite is bad - so you need to start with a water change to get the Nitrite out of the system. It doesn't matter what your fish is dealing with, dectecable Nitrite is going to compromise its immune system and make it far more difficult for the fish to fight off whatever its dealing with
If you answer every question we ask with 'fine' or 'normal' you need to realize that there really isn't any help that we can offer you
Lets start at the beginning..
Temp?
Salinity?
Ammonia?
Nitrite?
Nitrate?
Ph?
All other inhabitants?
Everyone else is acting okay?
Any new livestock?
Any new equipment?
Any changed / serviced equipment?
On a side note, photos are also very helpful in determining what is going on. If you can't get a photo of your actual fish, try to find a photo of another fish with the same condition and post it online