I'm a little leary to jump in here with all the talk about leaving fish in the DT and using garlic and such, but here goes ...
First off, Tangs are one of the most susceptible fish to marine ich, so no surprise you have it
I'm a proponent of both a QT regime, and a hyposalinity regime for cases of MI
I agree with the idea of leaving the fish alone so as to not stress them anymore than they already are, and keep them fat and healthy to fight off infections, but MI can apparently cycle up to (if I remember correctly) 11 times before it dies out (or is that 11 months ? ...)
In either case, it's a long time to go without adding
anything new to your tank if you want to be MI-free
If you want to rid yourself of it, go fallow (read below)
Healthy fish can fight off most diseases, just as you and I do, but the simplest things can kill very easily
MI can attack the gills just as easily as the outer visible parts, so watch out for heavy breathing issues
On the flip side to your guys having it, many cases of people losing their fish to 'MI' have in fact been mis-diagnosed and were actually Velvet. It kills, and
fast
So some of the horror stories we've read may not in fact be about MI at all
And there are lots of reefers (even here on Canreef) who actively have MI in their tanks and live with it
I've had fish in QT with MI (bad, bad coverage all over) and it's never killed anything I've owned, but that's during a hypo treatment so the effects of the low SG may come into play under heavily infested conditions
I can't say I've ever had any experience with long-term exposure in a DT as I've never allowed it in
So, with that said, it's up to you what to do next
You can either live with it now that you have it, or remove them to a QT for 9+ weeks (11 is better) while you leave your DT fallow and wait for the MI to die off
Keep the inverts in the DT and 'ghost feed' the tank to keep them alive
Then in the future use your QT everytime you buy new critters
Either way, I'm still gonna say it ...
Get a QT going and use it for new critters
Good luck, and Happy New Year !