IME there is not much that can be done. In general fish get sick when two things happen, 1-the pathogen is present AND 2-they are stressed. You can treat for the pathogen, but if they are not stressed they will not get sick. On the other side, if they are stressed you may treat this pathogen, but they will get sick again unless you remove the stressor.
I have talked to a few people who have used New Life Spectrum's Thera+A to treat marine white spot. It has enough garlic to kill some parasites.