can u specify aperture,iso and any other filter if used?
How are you lighting the fish and coral to give them that black background.. When doing regular photography, I typically under expose the background and light the subject to get that look . In some cases I need to grid or flag the light to prevent the light from spilling into areas that I don't want lit. I e never tried lighting aquariums other than with the tank lights.
Wow. Amazing. Ridiculous. More!
When I do fish photographing , I often do it with flash. Then you often get the black for free. When I photograph corals I use the help of Light room.