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 of course internal parasites. So the treatment is not a one size fits all I'm afraid.
What are you feeding?
I would follow this schedule in treatment using either API'S General Cure(first choice) or Tetra's Parasite Guard, also make sure you remove any carbon source prior to any treatment. I also don't recommend feeding during treatment to allow the digestive track to clear and leave the overhead lights off this allows for a reduction in stress.
Day 1. Perform water change then treat at full dosage.
Day 2. Perform a 30% change and re-dose at full strength.
Day 3. Perform a 30% change and re-dose at full strength.
Day 4. Don't do anything again.
Day 5. Perform water change in excess of 50% and re-dose at full strength.
Day 6. Don't do anything again.
Day 7. Massive water change in excess of 60-70%.
Day 8. Feed and see if everyone is eating.
The last thing I would add is Epsom salt dosed slowly at 1 tablespoon per 5 gallons, then replace the only the salt you've removed from the water changes.