Elaborate - higher salinity causes less dissolved oxygen?
Higher salinity means more trace elements per gallon, so corals are usually happier.
Fish have an active way of regulating their osmotic pressure and therefor don't care as much for higher levels.
Lesser developed animals such as anemones, etc have a bit more of a problem to adjust to "unnatural" levels.
This. If we are going to go to all the trouble to replicate natural seawater in terms of trace elements that are in it, it doesn't make any sense to me to use less of</em> it than is found in natural seawater. Fish aren't an issue, but corals, being much more delicate, will be much happier with levels approximately that of nature.