I know the one time I used Cupermine the recommended level was .5. Anything really higher was toxic and lower was far less effective. Make sure the brand your running is doing what your hoping its doing at .3. Also keep in mind if you plan on using that tank or selling in the future, copper can remain in there. Not sure of the process of getting it out or if you even can. I'd hate to see it leach out in the future on you or someone else. For the QT, I would not let anything from the DT near it. I'd use new water only. Even tools, cleaners, feeders, etc, keep everything separate. (even water change tools)