i usually use 10 gallon tanks for TTM, then i have a 20 long that was a long term QT tank, that i am now calling grow out.
plans are to reconfigure the grow out system to be two larger tanks linked in to the same sump. hopefully coming soon...
i keep a few other tanks around dry just in case, including additional 20 longs. in the event i need to QT a larger fish, or need a temporary home or hospital set up.