Aussie diver,
You asked for feedback. Well, it's a very nice pic, with good focus and exposure.
If it were mine, I would crop it to draw the viewer's eye to the picture's main reason for being.
Any book on photography will tell you that the viewer's eye is normally drawn to one of four points in a frame: one of the four internal intersections of a tic-tac-toe grid covering the frame.
Imagine the shot is divided into nine equal parts, divided vertically and horizontally by two lines each way like a noughts and crosses or a tic-tac-toe grid. The golden rule is to try to put the main subect of your pic on the intersection of two of those lines. That is, a third up from the bottom and a third either left or right from the edge, or a third down the way down from the top and one third left or right from either edge.
This style of composition makes for a much more interesting shot than the standard technique of putting the subject smack in the middle of the frame. How many boring shots have you got of someone's face right in the middle of the photo? For a portrait, try putting the face a third the way up, and a third the way to one side depending on what else is in the pic. For a landscape, try composing the shot so the horizon is a third the way from the top, or the bottom and see the difference.
OK, back to your shot. Most of the action is in the top and left hand side of what you've shot. Why not crop the shot to put the fish's face about a third the way from the bottom and about a third the way in from the right of the frame? In this pic, you're only losing some boring black sea to the right of and below your main subject - the lion fish's face.
I tried cropping your shot, but it's very small already so I won't try explaining by example. But shifting the fish's face down and to the right could be worthwhile.
