I recently purchased some new ballasts and bulbs for the 400g. I've been observing the livestock and debated whether to change the current configuration to only 250w bulbs or to upgrade all the way across to 400w bulbs. For the past 10 years, I've been running 250w - 400w - 250w bulbs because the center bulb was always a 20,000K bulb and the outer bulbs were 10,000K. This worked nicely, and it looked like the sun was shining down in the center. PAR was equalized since the bluer bulb's lesser intensity was punched up with the additional watts.
However, 10 months ago I installed TwinArc bulbs which are 10,000K for a few hours and 20,000K for a few hours and what I ended up seeing was a hot spot in the center of the reef. Daily. I had two choices: Change the center bulb to a 250w bulb and ballast, or change the two outer bulbs and ballasts to 400w. With the footprint of the reef being seven feet long and three feet wide, it seemed the better choice was to upgrade to 400w and give the corals a better bath of light.