I would agree with david if he looks like he has patches where the blue is gone. if it is external brown stuff then maybe a fungus. If it is patches of faded blue tissue then lateral line is a good possibility. make sure you are feeding algae once a day, check for stray voltage in the tank and make sure you are feeding foods that are rich in omega
Best bet is to assure there is no stray voltage then make sure you are feeding him a good diet. Are you using dried seaweed? If you're feeding flake and meaty foods every day you need to incorporate more algae into the diet.
Take some advice from the ghosts of half the oceans population of powder blue tangs.
Susan knows nothing about keeping us healthy.... I was the last and most recent victim. I came straight from Donny's big fat and healthy.. within 1.5 weeks I looked like she had rolled me salt.... then she takes and puts me in complete darkness in about 5 gallons of water.
The Ghost of the Powder Blues
P.S. My buddy Mr. Sohal got wind of this after a down payment was mad on him and jumped out of the tank to save himself from the slow death and torture of Susan's tank.
HEHE! guess what susan, Brandon is the only one who knew about you sticking that last one in your sump! Makes you think doesnt it! And I doubt david knew about the sohal!
Im just sayin!
Please everyone at the stroke of midnight tomorrow night turn off all your house lights and leave only a lone moon light lit on your aquarium
There will be a 60 second moment of silence in remembrance for all powder blue tangs.....oh and for Jimmy Jumpy the scared to death Sohal tang...literally.