The fish looks like some kind of sweetlips (Plectorhinchus sp.), and since you're in Japan, depending on where it was collected, it may be a temperate-water species, which means that it should be kept at cooler temps (70*F or less). The fact that it's eating is good, but if the fish is indeed a sweetlips, they get large and lose their juvie markings to become rather drab-looking adults.
You mentioned that the fish eats everything...does that include other fish? If so, I COULD be some kind of grouper/seabass.
I do agree that you should return it if you can, get some LR into the tank, and do some research on the type of fish you'd like to keep...