You might also try using beta-glucan, another immuno stimulant which is supposed to help fishes immune system. I know I have read some studies on it but can't find the links on it. You could either buy it and add to the food or use a fish food that already has it in it such as formula I & II flake food. OceanRider.com has a food supplement called Vibrance which has, among other things, beta-glucan in it or you might find some at health food stores.