OK... I'm not agreeing with the advice the LFS
gave, but I don't think their advice was really that stupid as folks here suggest.
Many books and LFS
work under the impression that a lower salinity (around 1.020-ish) makes life easier on a fish. That's why you often see recommendations for lower salinity in FO
tanks. It gets down to the amount of energy the fish has to expend on balancing the osmotic pressures and pH levels between its body and the surrounding water. Lower salinities are thought to give the fish more energy to fight off diseases, etc since they don't have to expend that energy to just plain stay alive. I don't know if this is true or not, but I've read it enough times to assume there is some truth behind it somewhere.
There's also the fact (?) that fish can deal with a quick lowering of salinity better than a quick raising of it. The hyposalinity instructions I have followed in the past basically take the 1.025 water down to 1.009 in about 2 days - but then take about 10 days to bring it back up. Things seemed to be just fine - the fish endured it, and it's still alive to this day.
So the advice the LFS
gave was essentially to lower the SG
from 1.025 to something around 1.020, probably to help the fish fight the ich. If there were no inverts in the tank (maybe they didn't know about the duster), and it was just fish only, this advice isn't as bizarre as it seems. Not necessarily what I would've recommended, but it's not like the LFS
was just making stuff up. LFS
are people too!