Stringy white feces is a great indication of a digestive issue but it does not always mean internal parasites, what your actually seeing is the lining of the intestinal track. This irritation can be caused by stress, improper diet, low quality foods, and of course internal parasites. So the treatment is not a one size fits all I'm afraid.
As mentioned if its truly spironucleus vortens or other parasites then ingestion of any medication is by far the most effective treatment, this can be medicated flakes from Angels(my first choice) because they are soft and easily digested which the fish prefers over a harder pellet(even soaked still harder than flakes), pellets soaked in a 3% Epsom salt solution, or a water born treatment such as Clout if the fish is refusing foods.
I would not worry about medicated foods or other medications at this time if they're all eating, watch the feces and go from there in my opinion.