The fish is a 'Golden Pilot Jack' aka Golden Trevally - Gnathanodon speciosus
Smartorl has helped a lot, so some of this might be repetitive.
Pilot jacks are insane swimmers. As juveniles, they often pair up with sharks, whales, etc and swim the open ocean, thousands of miles at a time. When small, they have a vibrant gold coloration with vertical black barring. As they age, their body starts to turn mostly silver, some gold still left here and there, like in the fins.
Some possible reasons why your Pilot isn't doing so well. Pilot jacks are a strong schooling fish, they live in large groups from 20 to over 100 fish. Keeping a solitary one in an aquarium probably is helping to increase stress, since there isn't another Jack to shoal up with. Second, it's hard to tell from the avatar, but it looks like the fish is already pretty skinny (how he looks pinched above the lateral line), and is that right eye cloudy? Pilot Jacks are large, active fish, that need feedings regularly. From the time this fish was collected to the fish store, he probably hadn't received a decent meal, and by the sounds of that LFS, probably didn't get one there either. With larger fish that aren't plankitvores (fish that eat small bits of food in the water column), I like them to be eating large pieces of foods. Whole jumbo shrimp, scallops, silversides/lancefish, etc. This makes for a far messier approach to feeding, but it provides for more exercise for the fish, rather then simply gulping as much food as they can.
Additionally, to put it bluntly, Jacks just aren't that intelligent of a fish. I think a large Angelfish, like an Emp, or something, has about the same brainpower as a school of jacks. They are a big dumb fish, with a big mouth. What tankmates do you have him with? If any too small, I wouldn't be surprised to see smaller fish start disappearing. And finally, the obvious elephant in the room, tank size. Personally, these Pilot Jacks simply don't belong in the hobby. IMO I wouldn't stock Pilot Jacks in anything less then a 10ft long, 3ft deep tank. The height of a tank doesn't really play a role for most swimming fish. They swim laterally, not up and down. There is also something to be said for the fish being older. In my experience with larger fish, there are distinct personality differences between say a 8" Pilot Jack, that was collected @ 8", and a Pilot Jack that was collected at 3" and has grown to 8". Adult, or older fish, never seem to adjust to captivity as well, or as quickly, as their younger counterparts.
