no flash, and quick shutterspeed. do what you have to do to get that quick shutter speed!
the above pic isnt perfect. "DOrthy" (if you have kids you know Dorthy from Elmo's world, ha ha) is always moving and carrying on. I "stopped" her with a high shutterspeed (which for me meant high ISO)
you can also use a "panning" technique (hope I spelled it right) and follow the fish as it moves with the camera...
on a kinda off topic note, I would love to see your saltwater fish. someday we may go to saltwater. I love the fish and live coral!