Agree about the lighting of the fish. Tone that down very much. The leftmost fish is burned through. Then, you don't get a sense it's a fish-tank. Enclosing a scene in a cube with a water mat applied might help. Also, it would be nice if there was a room or something wisible behind the sub. Putting a thin cube in front of the camera with a medium glass material applied would make it more plausible, but would also increase the render time. I think you need more specularity on the fish and the water weeds. And put something close to black in the Specular. You'll get a wetter look. BTW, rotating the skalar and the koi along the Y axis, so more of the bodies is visible, might help, but I'd have to try that to see. Also, more transparency on their fins would make it more plausible.

-- erlik