I lost my 4 year old volitan lionfish just over a year ago when the same thing happened. It stopped eating like you've described and was dead in a month. I blamed the size of my tank (120gal) for a lack of anything better to blame. Other inhabitants were: snowflake moray, tomato clown, copperbanded butterfly, and purple tang. I had mostly fed it krill and silversides often soaked in either selcon, vit chem or garlic. Maybe try offering it some live food if you can keep your other fish from eating it first.
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