yellow tang with rusty blotches?

Here I go again. This time it is our 75 gallon. Background: 75 gallon; 60 lbs. live rock, assorted clean-up crew, a pair of tomatoe clowns, a tibeken angel, a yellow tang. The tang and the angel were added after the tank had COMPLETELY cycled. They have been residents for approx 3 months. The clowns were added about a month later. All have grown nicely and have voracious appetites. They seem to get along famously as they are almost always in close proximity of each other. We run an Aqua Remora Pro skimmer and a cannister filter, two power heads. The cannister is cleaned every 2 weeks. Water test show 0 ammonia, 0 nitrates, 20-30 nitrate. I have been dosing with ESV Bionic 2 part system to increase my calcium. As a kick start I have also used Kent Marine Liquid reactor. Salinity has been fluctuating between 1.021 and 1.023. We seem to be experiencing a lot more evaporation lately.
Now the problem. Yesterday the tang developed rusty colored blotches. They appear to be mainly concentrated where the fins meet the body. He shows no other signs of distress. I do not notice any rubbing or other signs. He is eating constantly as usual. He gets a mixed diet . Any ideas?

hemorrhagic septicemia

Thanks so much Orion Girl. That was right on the money! Since I do not have an ammonia spike I must suspect a pathogen. I checked Drs. FosterSmith web site and they have several non-prescription antibiotics. The treatment calls for a broad spectrum antibiotic. I have to choose one. I will treat the whole tank if possible since this is something I would not see on the clowns or the tibeken angel. Will the antibiotic harm sea mat corals? That is not mentioned in the literature about the drugs.

Thanks again. I felt rather foolish when I realized the obvious--the medication would kill all bacteria---duh! Evertime I think I have graduated from the "newbie" rank ( after all I hit 100 posts) I make a stupid statement. I conferred with Foster-Smith rep and asked for a gram positive, gram negative antibiotic and they suggested Furanase. I ordered this tonight. Wish Tommy Tang and I some luck. He has a rough road with a novice owner.