Mike- When taking pictures of a tank it is always better to take them when the tank lights are on. Taking pics after the lights have gone out, and using a flash, really washes out the colors of the corals.
Try to take most of your pics when the tank lights are on, you will get much better pics.
If you have to take a shot at night, don't use a flash. You will still get better color, even though not the best.