Actually when soaking pellets in a 3% epsom salt to distilled water this will certainly cure most internal parasites including spironucleus vortens. Digestion is the key here when feeding, but the problem with alot of fish suffering from digestion issues is that the fish refuses food. If the fish the large enough you can actually use a pipette with a soft nozzle to force feed this solution directly into the gut. Epsom salts not only cure the parasites but ease the pressure in the already irritated digestive track.
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 improper diet, low quality foods, and ofcourse 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 me medicated flakes(my first choice) because they are soft and easily digested which the fish prefers over a harder pellet(even soaked still harder than flakes), epsom salt soaked pellets, or a water born treatment such as Clout if the fish is refusing foods.