IME acro SG 1.023, they werent happy. Hardier SPS like monti & pocci 1.022 & unhappy. But only for 1-2 days.
Google says reefs are naturally found at 32-40 salinity (SG 1.024-1.030). I think shrooms and softies can handle more extremes and more swings.
Most living things, including humans, can handle lower SG (less salt) so water goes into the cells and swell. High salt means water comes out of the living cell and shrinks and dies faster. I read a research paper that said basically the zooxanthellae stop photosynthesis more at higher salt. coral have no organs im aware of to regulate internal cell salt or water concentrations.
What happens in the wild and what we can select for are 2 different things. Im sure there are strong/adaptable acros out there that can be selected to thrive at lower SG- but why? Fish are adaptable...