Agreed with the above. I've nothing to add to that, so I'll just throw out some food for thought: tolerance to extremes in salinity vary hugely not only by type of organism, but certainly by species, and even among populations.
For instance: a study from a few years ago (the citation escapes me) looked at effects of hyposalinity on GBR corals. Normal salinity was 37 ppt around these corals (s.g., ~1.028). A drop to the lower 30's ppt caused low level bleaching in many corals, and killed some within a few days. A drop to the upper 20's ppt killed some within hours.
In contrast, a friend looked at the effects of hyposalinity stress on Siderastraea radians (which is a somewhat "weedy" species) collected from Florida Bay. Populations from areas that usually saw a salinity of ~35 ppt and rarely saw a salinity below 30 ppt did more-or-less ok down to a salinity of at least 20 ppt, but started to really have trouble and die in the teens. The same species from different populations that regularly saw hyposalinity stress (down to 15 ppt) did fine down to at least 15 ppt and didn't begin to fall apart until down around 10 ppt.
Not many organisms are that forgiving. As with the GBR corals above, salinities within the range people sometimes maintain in captivity are enough to really harm some animals. It's best to err on the side of caution, and shoot for a target that is likely to be acceptable by everything we might keep.
cj