Cory Turn For The Worse....(SOLVED)

Once treatment is finished, I tend to move across the fish from the hospital tank into the main tank after the main tank's next scheduled water change, when the water will be at its optimum/cleanest, which of course is what your fish will need.

Also, beforehand, I've also tended to have a day or so of gradually diluting the hospital tank's medicated water by replenishing it with clean water and even some water from the main fishtank; this way, the fish ought not to find the transition from medicated to unmedicated water too difficult and it reduces the likelihood of medicated water getting in the main tank (where it could be harmful to the filter bacteria and any invertebrates in particular). Running some carbon in the hospital tank's filter also helps, and I tend to leave the carbon in there after the fish is transferred (for 24+ hours or so), plus have a few days of having the filter running in the hospital tank with an overdose of dechlorinator in the water, in order to decontaminate the equipment (including nets, decor, etc) of any copper or other medication ingredients.

Great news.