zoanthids are notorious for doing this without any reason. They can do it if the alkalinity swing too much, the ORP swing too much, etc...
I once put Kent "iron and magnaneese" in my tank with the recommanded dosage and some of my zoanthids instantly shriveled and they died in a day or two after that.
So no more dosing for me, except by the drop to make sure things are in normal quantity.
I dose vitamine C for many months now and it does help to reduce those melting episode.
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More fish die from human stupidity than any other disease...
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