ISSUES!!!!!!!

If the fish is still eating I would say the best course of action is to simply feed him like crazy for a couple days and see if the condition goes away on it's own. This is farily common, small breakouts will often go away without trouble and fish frequently develop immunity to small populations of termatodes.

 

Here's my thought:if you have one fish that's a little sick and you uproot everything in the tank you're going to stress everybody out and then you may well have a bunch of sick fish. Just take it easy. Give it a couple days and see how things are. I've been right where you are and even though I wanted to do something drastic I was helped by a few good, experienced reefers and all turned out fine.

Feed the fish. change water. Breathe.

 

In my opinion I believe you are doing the right thing by treating all of your fish in the QT process. If you do not rid the tank of Ich it will always be present. You may not see signs of it but eventually you will want to add other fish which will bring stress to the tank and once again your tang will probably show signs again as well as the new fish could come under attach. It may be easier on the fish if you wait until the signs subside before you transfer to a QT tank but if you don't do this you will always have the potential of an outbreak.

It sounds like you have the means to set up a QT so for you it is a good choice, not everybody has the equipment or space to do so. From my experience in doing what I am suggesting my Hippo and Sailfin are just great. I will not lie, I did loose my powder brown but I can say it was not due to Ich.

For future additions to the tank, both fish and corals, it would be wise to dip corals and QT the fish to prevent adding new diseases or parasites to your system. If you want more information from a more reliable (meaning scholarly) source check out this link. He goes into great detail on this topic and will give you more information than you will know what to do with.

Marine Ich Myths & Facts: http://www.reefsanctuary.com/forums/fish-diseases-treatments/23132-marine-ich-myths-facts.html

Curing Fish of Marine Ich: http://www.reefsanctuary.com/forums/fish-diseases-treatments/52236-curing-fish-marine-ich.html