Just remember that if you do use copper in the tank, you can't use it for anything besides a quarantine tank. The silicon will absorb it and it will leach back out for a long time. That's no big deal really. A new 10 gal. is only $10. Just make sure to provide lots of oxygen. That will help a lot, especially if their gills are infected. It is kind of a pain though, because it causes salt creap.

You might want to keep it set up as a quarantine tank. A lot of people put a black molly in it to keep the biologicals going when you don't have a fish being treated or quarantined. Just acclimate the molly slowly. They are sold as a fresh water fish but can tolerate brackish water well.