What do you want to quarantine it for, ich? If so, it's rather pointless as it will be in the DT and just get re-infected when re-introduced. Another removal and QT process will probably be no better on the fish stress wise than simply leaving it in the tank.

Hey Joe
I prefer to put an Ich infected fish in QT until the visible spots disappear, then start the clock. Minimum 4 weeks to pass the next cycle of Trophonts, then watch for another 2 weeks for signs of new infection. So in my case, it can be up to 7-8 weeks in QT.
Last time I had an issue I ended up running my tank fallow for 86 days while I treated the survivers in a Hospital tank.

Good luck bud

Just for interest sake.

I read some interesting research where they say if you have ich in your tank and for reasons you can't remove fish or treat tank. If you wait 11-12 months without adding anything new-no corals,fish rock etc the ich will actually die off naturally as after multiple generation cycles it wears out. Genetically it just weakens to the point where it can't survive.

I've heard that too, but with a new strain taking up to 72 days per cycle, that 11-12 months just became around 30 ...

What was never mentioned is tank size? If the tank is only 50 gallons ish..... then the stress of a small tank will continue to bring on disease..... may not be ich at all. Tangs need space or they will not ever be healthy and happy.