Use Jungle Ick Guard or Wardley Ick Away. Be sure to use Ick Guard II on small fish, fish with no scales, or cats. Take out snails, amphibians, and activated carbon filters. Adding any type of goldfish (even a healthy one) to a tank with tropical fish is an invitation for ick. As weird as it may seem, the feeders may have killed your cat.
Use standard aquarium salt to prevent re-infection. On fish that have severe cases, you might try to remove in manually with a lint-free wash cloth. Use ACE or Start Right to calm the fish down when you have your hand in the tank taking off the parasites.
Good luck and I hope things turn out okay!
I wish I knew as much as I act like I do...