[DanMan36]

He has white and dark (black) little dots all over his body, and his cheeks are discoloured...Diagnosis ?

[leebca]

Imagine calling up a doctor who doesn't know you and saying, over the phone, "I'm sick. I have spots over my body and my face is discolored. What is your diagnosis?" This is very challenging.

My point is that without a photo and considerable more information it is nearly impossible to help. The symptoms match about a few dozen common ailments not to mention a few dozen more, less-common ailments.

Can you post several clear photos of the fish and each feature you are talking about?

Some background info would be good, such as:
How old is your tank? When did it originally cycle?
What is the size (dimensions and gallonage) of your aquarium? Does the gallonage include the volume from the sump and any refugium?
Do you use carbon, skimmer, mechanical or other chemical filtration?.
List all specimens & sizes in the tank (fish, inverts, corals, clams, snails, crabs, shrimp, etc.).
Do you use a quarantine tank and procedure?
Foods you use and feeding schedules.
How long have you had this fish? If the fish was recently acquired (6 weeks or less), two more questions: Did you treat it or give it a dip before it went into the aquarium? How did you acclimate it – what procedure?
Do you use any vitamins? Fat additives? Any elemental or other additives? Please list all.
Chemistries – you gave none. Do you test for Phosphate, alkalinity, calcium and magnesium? Please give actual current numbers for no less than: ammonia, nitrite, Alkalinity, calcium, magnesium, and anything else you test. Have these numbers been changing lately?
Water parameters – please give actual numbers (pH and your pH range, salinity or sp. gr. & range, temperature range)
Do you see any of the following in your system: hair algae; micro algae; cyanobacteria growths (red slime algae); dinoflagellate (zooxanthellae) growths; brown algae; diatom growth; slimes; off-colored patches on rock or substrate that are not coralline; etc.?
Water changes (how much and how often).
What is your source water? (Tap water, RO water, DI water, RO/DI, distilled, etc.)
List what you added or taken out of your aquarium system (living, decorations, and equipment) during the past 6 weeks.
Maintenance schedule. What have you done lately?

[DanMan36]

Here's a pic

[leebca]

The photo shows a severely under nourished fish. It has symptoms of being attacked by tank mates and/or deteriorating due to bacterial-infection.

The photo isn’t close enough for me to be sure, but the fish may have a Black Ich infection. Yellow Tangs are prone to getting this. Search the Internet for photos of fish with Black Ich and compare them to what you are looking at – see if there is a match.

This link has a photo reference and provides treatment guidelines IF you believe the condition is Black Ich: http://www.petsforum.com/personal/tr.../blackich.html