Usually you just treat the affected fish and then watch the others for signs of trouble. Any time you can keep from using medication on a fish they are better off so if only one of the fish is sick try the safer approach and treat only him. That way if you need to use the same medication on the others later they won't be acclimated to it and it will have a better chance of working. Kind of like building up a resistance to penicillin.
One persons opinion. Someone else may have a differing one.
Rose
