Yellow Tang sick

My yellow tang is not doing well. He is normally a great eater and very active. He is only about 2.5-3 inches long. I have had him for a couple of months. He looks like he is gasping for air with his mouth open and having trouble swimming. Now he is lying on the sand breathing heavily. My tank is a 55 gal with only a few zoos and mushrooms and live rock. The only thing I have done is add a new t5 setup (going from reg coralife tubes) and an auto top off yesterday. My clown looks like he is fine and ate like normal but the tang did not touch a thing.
Ph: 8.3
SG: 1.023
Temp: 81 on the digital, 78 on the floating
Calcium: 440
dKH: 8
Phosphate: 0
Nitrate: 3
Amonia: no test kit
Salinity: 32.3

 

Well, one hour later and he's dead. My daughter's going to be sad when she wakes up tomorrow and "Yellow Nemo" isn't there.

 

man, sorry to hear. im going through a spell myself of dead fish., first was my powder brown tang, then one of my paired clowns, then a 3 stripe damsel, then my other clown, and now my goby looks like he is about to kick the bucket. I know the feeling.

 

It might have been gill flukes.