If the gills, and only the gills, were affected.....and since fish breathe through the gills by passing water over the gills.....it appears to me to be a water quality issue of one sort or another, be it a toxin or whatever. My guess is a toxin from the Sea Apple(s). To borrow a line from "Reef Invertebrates" by Calfo & Fenner: "Sea Apples (Cucumarids) are definitely more difficult to keep in captivity. They also include some of the few species in this class (Holothuroids) actually toxic enough under duress in practical applications to kill fishes (the toxin is harmful to few if any invertebrates). .......... It is very important to note that if the Cucumarid is ultimately stimulated to exude toxin, all fishes in the system are at risk of sudden death."
Sea apples are a big risk. Especially in a small system such as yours. I won't even keep one in my 180.
HTH and cheers,
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Mark.
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