jonnypro I agree with Animal-chin prolly need 3 units but I would think you would have to cut the top middle section to get proper lighting in the center these units cover a 2 ft by 2 ft section.
I was just in another reefing forum and a guy had only 2 of these units over his 72x24x24. Here is the pic. It was a frag tank also. The only bad thing I can somewhat see is the middle had all shadows.
newest addition to my tank with the new lights...... I have the lights at the lowest setting at this time.