Okay whats your water chemistry like?
how bigs the tank?
how long have you had him?
My guess is the lion has some sort of nutritional deficiency or that it has a clogged or weakened GI. Freeze dried krill is brutal on these fish. Silversides are good, but need to be supplemented w/ grocery store bought gulf shrimp (w/ shell on), chunks of scallops, crabmeat , octo or squid, stuff they eat in the wild. Also your feeding regamin is off, these fish should eat only a few times/week, it keeps ther growth rates down and prevents organ developments problems(Like fatty liver)
anyway what to do now i would do a bunch of water changes and get your chemistry perfect, are you considering adding antibiotics to this main tank or do you have a quarentine tank?