Okay, good. What kind of skimmer do you have?
As for the QT, if your tang is acting weird (what does weird mean), I'd start preparing to quarantine all three fish if necessary. How big is the tang?
If I were you, I'd get a simple 30 gallon tank (maybe a 40 long would be nice if you want), a heater, and a good HOB filter and get it set up ASAP. A QT needs to cycle just like you DT did, so it needs some time. I'd recommend quarantining every fish you get from this point forward for a few weeks to make sure it is healthy and is eating well.
Hopefully you haven't introduced any parasites into the tank, but I'd get a QT set up ASAP just in case.