That is ich (cryptocaryon) folklore. In order to eradicate the parasite naturally the salinity must be <16ppt (<1.009); otherwise, lowering the salinity is pointless other than therupeutic osmoregulation. If the copper concentration is correctly measured and kept at a constant rate then it should help parasitic infestations from constantly reoccuring, but there are downsides to this method as well. Regardless of an lfs
' "quarantine" or "expert care," I would always quarantine separately yourself and consider any fish sold as succeptible to a repeating parasitized system.