I normally recommend feeding a anti-protozoa medicated food IF the fish is eating, if it's refusing foods then a water treatment must be utilized as follows.
If the fish is still eating NLS pellets then you can soak them in a 3% Epsom salt solution and feed that exclusively for 10 days.
With regards to the stringy white feces I would go ahead and treat for digestive issues. The best medication for this in my opinion is Clout (this will stain silicone for short time). Follow this schedule and remove all carbon from the filtration as this remove and medication.
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%.
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.
The entire tank must be treated as they have been exposed. The last thing don't feed during this time as the digestive track must be clear.
Best option if they're eating.
Medicated Flake Food for Fish
Second best option is soaking foods in Epsom salts.
Or if they're spitting or refusing foods.
Amazon.com: Sentry AQ Mardel Clout Tabs, 100ct: Pet Supplies
Then the cause of the digestive issues need to be addressed, the most common cause is a poor diet. I'm not going to argue about what your feeding fish but low quality or foods that are hard to digest are the biggest issues.