My pure opinion, not based much except gut feeling. I don't like hypo, never have. There is too much room for error. The study you posted is 28 years old. This, in itself, doesn't mean it isn't valid. It well could be. However, new info on ich is appearing all of the time. The longer periods required for fallow tanks and the SG for killing ich with hypo has changed several times during my time in the hobby. I think its very possible that some strains of ich have been "in captivity" for generations and may have developed tolerance for common treatments. If so, strains of ich could have tomonts that could last longer than the norm in this older study. IMO, there is no reason not to think that ich could adapt to treatments after 28 years; of course, this would apply only to ich that had been around in the hobby for a long time and I think there probably are such strains, given the number and contagious nature of the parasite.
FWIW I know a Parasitologist (definition: "One who studies ex-relatives of Mr. Tuskfish") at a SW aquaculture lab who is constantly trying to find emerging evidence of ich resisting copper, so far nothing. He plans to do the same with one of the quinine drugs. Sooner or later, his results will be published, even if he finds nothing new.
