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Hyposalinity is when you adjust the salinity to below NSW. Usually 1.009 I would not recommend this if you do not have a refractometer. Swing arms are not very reliable when it comes to keeping a specific salinity like that. Too high and the ich will not die and too low and it could effect the fish. Otherwise I find it to be much more reliable and safer than using copper. Remember if one fish has ich ALL your fish must be treated, corals and inverts excluded.
I am big fan of hypsalinity, it's really done the trick for me. Personally I've found copper to be very harsh, especially on certain fish. Of course as everyone else has stated, you'll want to perform any method in a QT.