Well, to tell you the truth, its going to be about 30. You have a large tang that is going to be thrown in with several other fish. They are going to need room if you want to try and keep aggression down and keep the tank stable during the treatment period [you're looking at a month]. If you had QTed fish prior to the problem, one fish at a time, you could have gone with a 20gal Long.
Whatever your choice, try to get a long tank rather than a tall tank. This will give you length, which is more room for fish and more surface area for gas exchange.
You don't need fancy lighting, or any lighting at all for that matter, so that can save you money. No substrate, but the tank will either need to be painted on the bottom [outside] or placed on a table so that there will be no reflectiveness coming from the substrateless bottom.
Keep in mind the tank will be uncycled. Another reason that a bigger tank is needed. Take a look at the FAQ Thread at the top of this forum for QT info if needed.
Good Luck!