Yellow Tang with Black Spot disease

[saltcreep]

"Black Ich" is very common for all yellow tangs, as most are hosts to the parasite in the wild. Generally if it show up in the retail trade, it is a sign that the supplier in Hawaii has been lazy and has not bathed the fish in freshwater. This will normally eradicate the disease quite readily. If it is in your tank, it will have to be torn down and one option is to scrub it with bleach (just don't forget to neutralize the bleach afterwards).