the best thing is to have a QT avail and remove and treat the fish individually.
i would venture a guess that diet is the issue also. try soaking some food in a soak type supplement. Warner marines is good (vitamin/HUFA). I would also use a garlic type soak. (brightwells garlic power is great)
i would stay away from kent products. since they were taken over by central pond and pet they have diluted the products..
