Yellow Tang w/Splotches Around Gills

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I added a small yellow tang to my 3 month old 50 gallon reef tank (w/7 corals, 2 feather dusters, 2 scooter blennies, 2 emerals, 2 sallylightfoots, 8 hermits and 10 snails). I don't have a Q/T and added him directly into the system (I took a chance and am now hurting!). He is sick with "brownish/greyish" patches on either side beside his gills. What are the implications of just letting him go...and die? How will his problems affect the tank? Fish? Inverts? Corals? It's a natural system with 65 lbs of live rock...masterfully set up! Please, experts, what are my options???

 

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My system has been up and running for four months...the corals (and other critters) are simply doing fantastic! I bought the tang at a local *****, he's small (2 inches from head to fin). I noticed a little "twitching" and "jerky" swimming when he was around. Then he basically stopped showing up except when I add a little brine shrimp in the water. The "splotches" are fairly symetrical and growing in size...around the gill area. The skin looks fairly smooth with no outer growth...the patches look like bruises on a yellow peach (if there is such a thing!). I've had him in the tank for two weeks now, and the "splotches" are getting more pronounced. He really isn't "jerking" like before...but I don't see him that often! Again...please "experts" let me know what I should do at this point! One more thing, from day one, he wouldn't eat any of the "greens" I gave him in the tank. Thank you for your help!

 

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Thank you for your counsel! The water is testing out fine (at least as far as my fish pharm test kit will allow...if you have any better ones I'll buy on your recommendation) with no nitite readings and very little nitrate showing. I change my water once a week (five gallons). Play the scenario out for me. How long do you think he'll live? What can I expect. If he dies do I take a part the rock to try and find him, or let the crabs have a "heyday". Again...even as I speak, everything is alive and "dancing" before my eyes...but no sight of the tang. Thanks for playing the scenario out. Look forward to your reply!

 

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If the disease is what you think...will the tang die and "float" or die and "sink?" What do fish do after death? What if I don't find him after he dies? Since I hardly ever see him now, I probably won't know if he lives or dies! In any event, can I purchase more corals while the tang dies?