Yeah, get a Nikon D70s. :lol: The faster shutter speed of a SLR is amazing. However, even a point and shoot works if you use the flash at a slight angle.
Or turn off the pumps for 15 minutes. The fish will become confused and unsure, and may stay more still, closer to the rockwork. Try to get a shot without the flash.
Or aim the camera at a spot the fish passes regularly. Put the camera on tripod, press the shutter halfway to focus on that spot, and wait for the fish to pass into the frame. Take the picture.