I do treat fish for parasites when I QT them
I learned from breeders and importers it is a good precaution
many times fish will carry parasites ..the relationship is symbiotic..the parasite needs th host to live so i doesn't kill the host.
however, stress may change the paramters and the parasites may get the upper hand on the host.
I do not treat for bacterial infections unless they show signs of bacterial infection.
however, I do not treat for ich unless this particular parasite shows itself.