Doesn't sound good. What are you treating with? What are your plans for the rest of your fish, they have ich living with them too and they are bound to get it. You have a HT, good. I'd use it as a QT on all new livestock from now on; a QT is absolutely vital for long-term success. IMO & IME.. You really need a better and faster way to cycle your HT/QT when needed; here's whatI do, I've posted it many times and know it is about foolproof.
"In regards to Qt cycling; I've done this for years. Get a HOB filter; I really like Aqua-Clear, they have a big sponge and last forever. Don't use the carbon or ceramic noodles that come with the filter. Also, have some extra sponges on hand, they're cheap. Keep a sponge in the flow somewhere in your DT. When you need a QT or HT, just use the sponge that has been in your main system in your QT filter---the QT will be instantly cycled. When done, toss the sponge and keep a new one ready in your main system.
BTW, Cupramine copper, used in a QT,will not destroy a bio-filter." A cheap, air-driven sponge filter will work fairly (but not as) well too."