Create a grey line AROUND the fish that is about three or four pixels wide. make this line as a selection and then point in on a new layer some highlights and shadows. Basically, everything above the fish (and slightly to one side) should be black, and below it (and to the other side) should be white. At least, that's how I would do it.
Another easier way is to select the fish as a new layer, then use PhotoShop's Filter --> Render --> Lighting effects. In the last box, which is used to render the effect of depth on a surface (OK, so I forget the name of the setting and I'm not at my Mac right now) select "Layer ... transparency". Position the light how you would like it and admire the effect.