i have the same tank with 2x250w and its to much. You should be fine with the single but need to look into the high par bulbs like the xm 10k or the pheonix 14k. The ballast and reflector have a major impact on if it will be enough and how the bulbs will look. If you are running an m80 with a good reflector you should be fine imo. My corals grow down, so two is to much, with the higher par bulbs and good reflectors. Both sets of bulbs need to go at the same time and run the vho's as atinics for 10-12 hours and the halide for 8 or so, during the middle of the atinic cycle. This will give you good color and decent growth.