I use Cupramine and a de-wormer on all fish in QT too. Controversial, but it works. Almost all fish have been treated with copper along the way. IME, PBTs handle Cupramine very well and its a safe, easy med. If you treat or not, I don't think 3 weeks is long enough. You need at least 4 weeks to go through the life cycle of most parasites. Be sure your QT is cycled or expect to do a lot of water changes.and never use an ammonia neutralizer (Ammo-Lock, Prime, etc ) with copper. A 15 gal is fine, you'll also need a heater and a few pieces of PVC (or similar stuff) for shelter. Here's something I've posted a zillion times, IMO & IME, the best way to keep a QT/HT cycled:
"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".