You will want one now. Compared to the potential cost of losing fish, they are a wise investment. A 20g tank, a heater, some pvc
pipes for hiding and a little filter and/or a small powerhead for water movement is all you need. Watch water parameters as this will start as an uncycled tank.
I agree with the info above...copper or hypo are your cures. Garlic supplements help boost immunity, but do not cure anything. Leaving your DT fallow for 6-8 weeks lets whatever is in there die off.
Keep an eye on your Tangs too. If you still have the Scopas and Hippo, they are next in line to catch this. Tangs are very susceptible to Ich and those two, being in a small tank, are probably going to be stressed. Stress lowers immunity, which opens fish up to stuff like this (especially Tangs).