Could it be something in your tank releasing toxins (ie. Sea cucumber, injured fish) I've read on Reef Central about people who were having mystery die off and it was found to be an injured critter releasing low enough lvls of toxins to not affect fish/inverts but could hurt coral.
It's a long shot in the dark and I've looked but I can't find the article on Reef Central to link
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 My aquarium is nothing but a smorgasbord for my cats.....
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