hey there...What is the length of the tank? if it is, then you want to be looking at getting two of the halide lamps. One lammp is good for 2 feet of tank length. Yes, you could use only one in a central position, but you well be ending up adding corals to central portion of the tank where the light is as there will be low light area's at each end of the tank. If you have two, the light is spread across the full length of the tank..you will be able to keep a good range of coral under that type of halide lamp
Bulb wise, i would switch the bukb to a 14,000k bulb as this is more white with some blue mixed in..Bulbs are personal preference though..The actinics are only there to promote the visual appearance of the corals..
Just my opinion
Niko