No, this doesn't fit. You can't put a massive change of salinity on coral. I've kept them for long enough to know how sensitive they are. Now since there is no freshwater alternative to coral, there is no fallback onto the 'kind' excuse.
Pre-flood fish where different in that the sea (when it was created) had no salts in it and where therefore fresh water, meaning that all the pre-flood fish where fresh water fish.
Then how did coral come into existence? There isn't enough calcium in freshwater for reef building. Fossilized reefs are not that uncommon.