I know how difficult it is to shoot fish (in a barrel or otherwise) as I've tried it mself and so I know how well you've done here. The detail on the fish in pin sharp and there's hardly any noise.
Wow! He's certainly the center of attention, isn't he? :) Really nice, crisp capture and great colors. I like that you got that neat-looking rock in the foreground too. I'm wondering if the background is just a *little* too blurry, though; It makes the fish look a little flat/two-dimensional to me. Regardless, I still really like this. I could definitely see this framed and hanging in my bathroom (which is done in bright colors and tropical-related stuff).