Yeah that's not enough light for hardly anything except mushrooms and zoas... Sorry. I would keep looking for something more powerful. T5s are cheaper and give you more bang for the buck than MH if price is the issue.
300 watts of MH on a 120 g fish tank, you really won't be able to keep much... Most things require a minimum of 4 wpg. Anemones, more like 8 to 10. I know, watts per gallon is not a hard and fast rule, but we're ballparking it here, guys (Freak...).