To properly diagnose parsites you need to either study fecal samples or perform a post mortum diseection. Those are the only sure ways to tell. That will let you know whether you are dealing with protozoans, or nematodes. therapies differ depending upon type of parasite. Metronidazole, Fenbenzadole, and dimetridazole are effective against many flagellates (protozoans). You will achieve the best delivery of these meds orally (through food).
Red, hanging threads can idicate a nematode of one sort or another. Red inflamed anus and vent are two other indicators of this problem. Levamisole Hydrochloride is effective angainst many nematodes. a 24 hour bath at 2mg/L followed by a large volume--at least 50% water change--repeated one week later usually works.
A cautionary step might be considered if you think you have an internal parasite issue.. Feed the fish medicated food. If it is still eating and does have a prasite problem this might be you last best chance of favorably resolving the issue. Metronadizole and praziuantal are not effect bath treatments for internal parasites. External yes. Internal no.
If you wait until the fish stops eating and begins to thin out then you will not be able to gt most meds to the gut of the fish, where the parasites are. Parasite Clear and other generally available products are virtually useless against internal parasite problems.
fecal samples are the best way to figure it out. if you are unable to perform the fecals do any local vets get involved with fish?