MH would turn a 6 gallon into the fiery pits of hell. People with large systems run fans to keep them cool because they run so hot, even hundreds of dollars spent on cooling systems!
That aside, I'm not sure where you'll find a bulbed unit for a 6 gallon tank that would support corals.
Thankfully, zoas don't need much light. I think you could easily get a pendant type fixture with something like a PAR38 bulb to keep them alive. That said, a PAR38 would be too much light most likely and want to downgrade it some.
EDIT: If this is still the Fluval tank, there are ways to mod them to get LEDs into them. It's been awhile since I've read about it so it escaped me when I first made this post. I'd give some links to such, but at work and can't access most information that I'd be able to provide. Check out youtube, this isn't something that hasn't been done before.