I assume that any wild fish from CA or SA, or any pond raised anything has parasites. My quarantine is 90 days minimum and treatment is automatic.
1) discus - a) intra-oral injection with Fenbendazole b) Praziquantel c) Metronidazole (in stages, not all at once!!)
2) other fish: Prazi & Metro (remember that some fish can't tale these meds, cats and/or plecos I think, probably others)
Fish I get from a known good source (local breeder) I do what Mike does - observe and treat as needed. You could always get a microscope, some slides and covers, and do a poop smear or body scraping and see what the fish actually has (would make a great presentation/workshop meeting - IMO). There was supposed to be one at ACA but I never found it.
Many people will take exception to my approach - I'm getting used to it!!
Just what works for me.
TMS