This is just out of curiosity, fishy is healthy. How do fish get parasites when they live alone with the same decorations and clean, conditioned water? I'm thinking about internal and external. Could it happen when you change the water? Could something airborne get in the tank? (Mine is covered but obviously has breathing holes).
There are a number of internal/external parasites that are always present in/on your fish or in the tank. They only become a problem when the fish becomes stressed and it's immune system is compromised. Since all fish become stressed at some point in their life, even isolated fish are susceptible to disease. Of course, people with a large turn over rate of fish tend to face more disease in their tanks.