Unfortunately, I don't have access to this article any longer but scientific studies have been done showing reduces parasitic reproduction at lower salinities.
http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2761.1979.tb00146.x/full
If I'm reading the abstract correctly and have done my math right, it is basically saying that at 77-86 deg. F, 30% of Cryptocaryon irritans tomonts ruptured at a salinity of 1.007 s.g. (assumes 1.0265 NSW and no temperature correction). Of course, this only applies to one particular parasite, and 1.007 s.g. is pretty low. Apparently, one could also run a tank at 99 deg F or 68 deg F while running around 1.013 s.g. and rupture 50% of tomonts. I guess if all other considerations for fish are ignored at the expense of killing up to 50% of Cryptocaryon irritans tomonts, we have our answer.