Personally, the concern is a lot less about a possible hypo-resistant strain (although I do feel I have come across them when I previously used hypo) - the much bigger concerns and why I view hypo as virtually useless for most applications is as follows:
1) Human error - it's very easy with evaporation, inaccurate measurement, inaccurate measurement instruments, etc to go above the hypo threshold which re-starts the 30 day clock.
2) Ich is childsplay relative to velvet, brook, uronema, bacterial infections, internal parasites, and even some kinds of flukes. Hypo doesn't reliably eradicate these ailments, although it can kill several kinds of stubborn flukes.
Copper has a bad wrap but with better measuring tools such as the Hanna Copper Checker and a much more reliable product with proper potency (consistent) such as Copper Power, we can easily maintain a 1.75PPM level of copper. We can also do this for 14 days and then transfer to a new, sterile QT which shortens the 30 day mark a lot for treatment.
We've also learned that many used Prime or UV with copper, which is often fatal. We've learned about biological filtration and now have reliable products that provide this instantly (such as Biospira) so ammonia is less of an issue. We've also learned that medications can often cause oxygen levels in quarantine to be dangerously low so we've learned a bubbler AND/or a small powerhead breaking the surface are immensely helpful.
With the exception of wrasse, lionfish, and puffers (although more of these do seem to make it through the new process with these new advancements) some previously thought of as "copper-sensitive" fish were actually ammonia and 02 sensitive fish, rather than copper-sensitive. Take most angels, for example. They do fine in copper outlined above, generally.
If you use metroplex AND copper in conjunction for that 14 days, and feed GC soaked food throughout treatment mixed with "focus" to bind it to the food, you will treat ich, velvet, internal parasites, some bacterial infections, brooknella, and uronema.
After the 14 days in the new sterile QT, treat with GC or Prazi twice 5-7 days apart and it'll clear out flukes, most of the time.
NFG can be used in the following 14 days after copper as well for any stubborn bacterial infections.
