I have know a couple people who say that they have hade good luck by using garlic. There are several manufacters that market garlic based products. Soak the food in the garlic product before feedings.
Ever notice when someone who has eaten certain food, garlic for one, it 'seaps' through your skin. Same with fish it changes the composition of their slime coat so parasites dont realize that they are fish and their life cycles ends because they were not able to find a host.
I personally use it when I QT, it is also a flavor enhancer some fish go nuts for the stuff. I have gotten several fish to eat garlic soaked food and but not eat non-soaked food when in the QT..
-nick