I had a similar issue with a lion fish was caused by a bacteria or parasite brought in by some used live rock I had bought. The fish were fine for weeks until a fluctuation in my tank caused an outbreak of ich and gave the bacteria/parisite it's in on my fish
I quarenteed the fish that survived and did a mala-fix treatment to the tank
I hope that's not the issue in your tank the survival percentage in mine was very low
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