I personally would be trying to darken the back ground more especially if your at 30m and you have a sunburst or at that depth a sun blob coming through. I feel that the Lionfish fills up too much of the frame and a bit of the low fins are right on the boarder of the frame.
As other have said a wider aperture like f8-f10 for better depth of field. I would be shooting at your lowest ISO and shooting a higer shutter speed or the highest you have which syncs with your strobes. Higher shutter speed darkens the background and I feel gives a better colour graduation when you have the Sun in the back ground. Regarding strobes try the same shot in TTL and then try manual strobes with 3/4 power and then full power. I prefer to shoot manual for wide as you know what they are going to do everytime you shoot.
Have you tried just shooting the 8mm Fisheye without the TC? Just have to get use to getting really close and having good strobe placement.
Write down some different setting on a slate and when your in a position to do so and take a few shots with those different settings and when your back infront of a computer decide what setting you like best and go from there.