Nice pic.
I don't use a tripod or shut off the power heads for fish shots - only coral shots.
Edit: Just a note for fish that don't sit still. P mode almost always seems to go to 1/60s shutter speed on mine. You may need to go to Tv or M mode to get faster shutter speeds. Also, with that lens, you may need to zoom out to get the bigger aperture since the max aperture varies with the focal length.