Im guessing your 120 is 4 feet wide. A MH bulb covers and area of about 2ftx2ft well. If you are going to use a single MH bulb over 4ft, put it in the middle, and have lower light corals at the ends of the tank, the highest light corals right in the middle.
I would give 4 NO actinics a try, see what it looks like, add more bulbs if you want it more blue looking, because the NO is really just for looks.
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