This is a rather old thread, but since no one answered copper and formalin can be used depending on the external parasite and what you have in the tank. Cupramine or Coppersafe could be used for copper and Quick Cure for formalin. A gentler alternative to Quick Cure is Paraguard which is a "blend of aldehydes, malachite green, and fish protective polymers." The malachite green also helps treat external parasites.
I fortunately have had no need for these, so I can't tell you which to get (or which is best). But I have Paraguard in my "first aid kit" and if I need a copper based treatment, I'd probably get Coppersafe since it's cheaper.