250w mh moguls, IMO, loose par after about 24"... For me I have measured that from the distance between the bulb and the deepest coral that has good color. I run 1-250 mogul on a 50g that has a nice birdsnest on the bottom(approx 24" from the bulb) that has great color. I have 2-250's (both mogul) on a 110g (30" deep) that I have had to move most sps to the top 1/3 of the tank to get good color....about 20" from the bulb. All bulbs are 15k. I am considering going to 2-400w mh setup on the 110g to achieve better penetration in the deeper tank. Just some of my findings, dont know why I have a 4" discrepancy between the 2 set ups though..