Now that you've started cooper, do not lower the gravity. Also, if your treating with copper, you will need a copper test kit to make sure to keep the dosage correct for the duration of the treatment or else it will not work.
You will also need some freshly mixed SW
for water changes while they are in the hospital tank because unless you cycled the hospital tank before hand, your ammonia levels will go up very quickly and you'll need to keep them down via water changes. When doing the water change, you will be removing some of the copper medication, so you will have to figure out exactly how much to put back in.
The reduction of salinity is used in a different method of treatment called hyposalinity. I would recommend reading up on that as it is much easier and cheaper then doing a copper based treatment for future reference.