I know you can use NO on saltwater tanks. It's used a lot on FOWLR
tanks since it's cheaper than lights aimed for reefs.
In your case you can get 3 NO bulbs @ 40 watts each for 120 watts on a tank that is ~18 inches tall which is good light. But you wouldn't be able to get 120 watts of NO flourescents over a 20 gallon tank. Do you understand what I'm trying to say? And anthelia is a good low light coral.
Edit: Your upgrading to MH
from NO? Make sure you acclimate your corals to the lighting since it's a big difference in intensity.