F number is in the F14-F20 range. I go between the 60mm f/2.8 nikkor and the tokina 11-16mm f/2.8. Shutter speed between 1/160-1/200. ISO always 200 with flash.
In terms of f-stop and shutter speed, I'm always changing. Even depending on how high in the tank the fish is, I'll use slightly different settings.