The most likely culprits to exhaust your fish with few physical symptoms (white dots) are velvet and brooklynella. There are strains of both that have very subtle physical symptoms, and they're rampant in the hobby. Still possible but less likely are ich and external flukes. Ich usually has visible dots, and flukes take a while to wear down a healthy fish.
For any of these parasites (velvet, brook, ich, flukes), I would run the same treatment.
Start with a 5 minute freshwater dip. If the problem is a sever fluke infestation, it will be obvious at ~3-4 minutes. If it's not flukes, the FW will help reduce the number of velvet/ich/brook present. FW will not cure any of these parasites, however. You fish may panic but you shouldn't.
Then start treating the fish in a hospital tank with chloroquine phosphate.
If you don't have CP, treat with copper. Copper doesn't work for brook. If you know it's brook, do a bath in formalin first. But velvet is more common than brook, IME.
There are excellent stickies and posts on how to do these dips, dosing levels, duration, etc. in the "Fish Disease Treatment and Diagnosis" forum.
