hyposalinity, no matter how it is defined, is not effective against ick, until it is down to 1.009, just to clarify
and most oceans are in the higher range(1.025 give or take)
lower salinity as mentioned can also be stressful for fish as well adn is much less otlerable for inverts such as corals and shrimp. so unless you are keeping afish only without lr and inverts or corals tehn i would not even consider lowering it below 1.020(and i peersonally would not go near that even, mine is 1.0255 normally)
hth,a nd tanks tang and others