Re: Powder Brown in QT
Thank you for your questions. More than likely, after having treated your fish with Cupramine at a concentration of 0.5ppm for 4 weeks, all traces of ich (cryptocaryon) have been eradicated. In my opinion, the flashing that you observed in your Powder Brown Tang was most likely its way of removing the "fluff" that you described having seen on the fish. Because it is gone, I would expect not to see any more of that behavior. Cupramine itself is very gentle to fish and should not cause any irritation whatsoever, so I feel that the spot that you noticed on the fish was most likely the cause of the flashing.
Though it would be safe to extend the treatment for another week, I do not feel that this is necessary, given the fact that you have already completed 4 weeks of treatment with Cupramine. I feel quite certain that the parasite should be eradicated and that you can return the fish to the main tank. Of course, please use your best judgement, as you know your fish better than anyone and you can actually observe any goings-on.
Let us know if you have additional questions and have a nice day.