Last week I had a yellow tang start to breathe heavily in qt. It was in a well cycled ,ammonia free qt tank with other fish, having been through tank transfer. None ofteh other fish had or have any symptoms of anything.
Initially , I treated for flukes with a fromalin bath and prazi pro .No relief,.
The fish was swimming head up toward the surface and lumbering, dorsal fin withdrawn . Then went to the bottom and over on one side;figured it was going to die.
I kept observing and noticed small red splotches along the juncture of the dorsal fin and the body and a few other spots. Hard to see but since the fish was isolated in a bare ten gallon at this point and since it's yellow , I was able to see the splotches.
I settled on a bacterial infection, gill disease, or hemorhagic septicemia . Blood borne bacterial infection. Don't think therer is any risk of contagion to other fish but I'm not sure o fthat.
Alternatively, I was thinking osmotic hemorahging but acclimation was solid and there was no way to treat that anyway.
Unforunately , it was the middle of the night when I had this epiphany about bacterial infection.. The fish was near death so I rummaged around and fortunately had some Furan 2 on hand but the expiry date was over a year past due. I also had API TRISulfa another braod based bacterial med which had expired in Nov of 2011, close enough. The fish was going to die, on it's side ,struggling to breathe , some discoloration in the face, so a somewhat stale antibiotic wasn't going to hurt.
After teh initail dose , the tang was still on it's side the next morning, still laboring to breathe , but sitll living. I reckoned,I may just have prolonged the agony. Looking more closely , I noticed the red line was gone . About six hours later to my surprise the fish righted itself. By the next day ,still underr treatment it ws eating . Five days later, it's a pig and very healthy. I'm moving it back to the large cylced qt tank today.
The lessons I learned were:
not to overlook bacterial infections'
the value of qt for observation and ease ofc apture and treatment applications,
and that bacterial meds are dynamite good when I can diagnose the problem correctly.
Sorry about your loss.