Hi Wendy
Paul B is right. There is very little you can't keep under VHOs. I agree that leaving any glass lids off will be vital with gas exchange and temperature control. But, in a standard canopy, you'll almost certainly need fans with VHO's. They run hotter than T5's, but not nearly as hot as Metal Halides. But, once again, I highly encourage that you sit down with some resources and plan what corals you like. It will save you LOTS of headache (if you don't already have one
) in the long run. I highly recommend reading back through the lighting section;
Choosing a Lighting System. It's close to the bottom of the page.