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We have known for quite some time that OST is an effective treatment for ciliated parasites. A number of excellent studies and articles have made that point, particlularly Lowry and Colorni. I would suggest that hyposalinity is no more time consuming or labor intensive than treating with copper and is certainly immensely less stressful to the fish. When coupled with adequate oxygenation and raised tank temperatures there is even some emerging evidence that hyposalinity may be effective faster than copper treatment. I would respectfully suggest that hyposalinity would be a more appropriate prophylactic treatment for ich with copper treatment being necessary only in cases where hyposalinity proves ineffective.
Are you aware there are hypo resistant strains of ich? I can pull up the full study on Google Scholar if you like, but below was taken from here: https://edis.ifas.ufl.edu/fa164
More recently, studies have demonstrated different salinity tolerances among strains of Cryptocaryon. Yambot (2003) described one Taiwanese outbreak occurring in sea bream Sparus sarba at a salinity of 5 g/L, and another outbreak in sea perch Lates calcarifer occurring at a salinity of 10 g/L. These two strains were successfully propagated in the laboratory at 7 and 10 g/L, respectively, and are well below previously documented preferred salinities.
Also, many times velvet is misdiagnosed as ich early on by the untrained eye, and once the fish is already in hypo it is very difficult (on the fish) to combine that with needed chemical treatment. A fish with velvet doesn't have enough time to raise the SG to begin treatment.
You also must use a perfectly calibrated refractometer at all times while doing hypo, and I recommend an ATO to keep the SG at 1.009. If the SG inches up even slightly, the 30 day treatment clock restarts. You also must continually buffer the pH whist performing hypo. Hypo certainly has its downsides/pitfalls and is not as "easy" as it sounds, if you do it correctly.