Nice, you're off to a good start! That BGK's detail came out well for being so dark, and in front of dark substrate. On a side note, I saw one of those for the first time just yesterday, very cool looking!
If you can, train the autofocus onto the eye of the fish. Animal photos always look better when the eyes are in critical focus. To get rid of blur or catch some action, see if you can get more light on the subject, so you can use a faster shutter speed.
To keep from going too far off-topic, here's a random Silver Dollar photo. Slightly botched white balance, hence the greenish color cast: