The mark II has AMAZING ISO capabilities... even at 6400, which I've shot at. I don't have to do too much custom white balance with this camera either... it picks it up beautifully.
The filefish pic turned out that way because of a couple of things. He was right up next to the glass and was getting some of the bottom display lights on him..... so he had a nice fill light. This light wasn't reaching back to the corals which were far far in the back, so they were not lit up the same. I also lucked out in that I got this pic at an angle. I was kneeling down on the floor looking at an angle up to the water surface. Usually this doesn't turn out so well because of the distortion, particularily from their abused panels. And, as Mark said.... the potrait lens helps.... I love that lens.