how far can i lower sg URGENT

A lot of LFS keep their tanks down around 1.017 - 1.018 without seeming to have any problems with fish; inverts I would say seem to prefer it a little higher. However, its important to note that recent studies have shown that apparently the ich tomites aren't killed until around 1.012 or so, which I would say is almost certainly too low. Values around 1.017 seem to at least slow the proliferation, but I wouldn't count on this alone to kill them off. Don't know if that's of any help, but there it is.
 
my salinity is 1.021 when my tank is fully toped off. After about 1 week of evaporation it is 1.022. But who is to know how accurate my hydrometer is
 
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