Make sure you have no sundial snails in the vicinity. They love zoas. Make sure you have adequate water flow to take away debris. I blow them off occassionally with a turkey baster. Have you added any fish or critters that can be crawling on them and annoying them? They don't like being disturbed on a regular basis. Sometimes our favorite zoas just melt away without any apparent reason. Pathogens, bacteria, Chemical warfare, etc.
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