Generally I think it has to do with movement. The background images while they are moving if the screen is panning, are stationary.
The fish are all over the place in a purely random fashion. You can't predict where they will move to next, and therefore the 3D software can't either, so it is always slightly playing "catch-up". Granted this is done in microseconds because of the computer nature of the beast, but it is still doing it, with a ton of polygons to have to deal with, so I think random moving objects sort of stress out the properties of the 3D effect.
At least that was always the way I thought of the process.
excellent explanation, now i see exactly what you mean. cheers for that, it makes perfect sense and seems totally logically.
i didnt understand the workings of 3d and by how you explain, 3d works independently of the objects on the screen. i was thinking 3d may work of taking snapshots of the whole screen and all objects, and then quickly refreshing an overlay, slightly off-set creating 3d image. this is the reason i couldnt understand why increasing the depth worked for some objects but not others.
i hope that makes sense...