So, i am an avid (but still moderately new to it) photographer. I have tried different settings for my camera - i have a Canon XTi 75-300 zoom lens, and a Canon Digital 28-80 lens. I've noticed that the normal stuff applies to taking pics, but it's much more intense and needs extra knowledge to do so. Lighting, Focal length, ISO and Aperture and shutter speed come into play. The slower the shutter speed the better, but if your zoomed in (at least with a shorter small lens) the fish is going to fast for it to process and the light isn't there to make the shutter faster. Hence, the ok looking photo with a fish that's blurred. But with a faster shutter speed, the same amount of light, it won't snap the picture because of it. So fish moving: I've had luck at 1/30th of a second on 400-800 ISO, with an F stop of between 3.8 or so and 5. Hope this helps!